Silver halide photographic emulsions and process for preparing the same

Abstract: A silver halide photographic emulsion comprising a dispersion medium and silver halide grains, in which the silver halide grains contain a silver halide phase containing 3 mol% or more silver iodide and the distribution of the silver iodide in the silver iodide-­ containing silver halide phase is completely uniform. Preferably, tabular silver halide grains having mean aspect ratio larger than 2/1 account for at least 50 % of the total projected area of the silver halide grains in the emulsion. Because of the completely uniform silver iodide distribution in the silver halide grains, the emulsion has excellent photographic characteristics in terms of sensitivity, gradation, graininess, sharpness, resolving power, covering power, storability, latent image stability and pressure-resistance.

Description: Field of the Invention The present invention relates to light-sensitive silver halide emulsions comprising a dispersion medium and silver halide grains useful in the field of photography and, in particular, those comprising silver halide grains containing a dispersion medium and silver iodide, as well as to a process for preparing the same. Background of the Invention Of light-sensitive silver halide emulsions for use in the field of photography, those except silver iodobromide emulsions have only a limited use in picture-taking photographic elements with camera sensitivity. Silver iodobromide grains which are generally used in the field of photography contain silver iodide in the silver bromide crystal lattice of the grain in a soluble limit amount or less, or in an amount of iodide content of about 40 mol% or less. The iodide content in the silver iodobromide emulsions has the following advantages (1) and disadvantages (2). (1) The latent image forming efficiency increases, the light absorbing amount (or intrinsic absorption of silver halide) increases, the adsorbability of additives is improved and the graininess is improved. (2) The developability is retarded and the chemical sensitizability is interfered with. Many studies have heretofore been effected in the development of silver iodobromide photographic materials with camera sensitivity, so as to augment the above advantages and to reduce the above disadvantages. The most important feature is which site (position) in the silver halide emulsion grains (hereinafter referred to as "grains") silver iodide is to be positioned. Duffin (Photographic Emulsion Chemistry , by Focal Press, 1966, page 18) states that in the case of silver iodobromide emulsions, the important factor to be taken into consideration is the site of the iodide. The iodide may exist mainly in the center of the crystal, may be distributed throughout the whole grains and may also exist mainly in the outer surface. The actual position of the iodide is determined by the preparation conditions, and the site (position) apparantly has an influence on the physical and chemical characteristics of the crystal. In a so-called single jet method in which all of both iodide and bromide are first put in a reactor vessel and then an aqueous silver salt solution is introduced into the reactor vessel to form silver iodobromide grains, silver iodide is first precipitated so that silver iodide gathers in the center of the grain with ease. On the other hand, in a double jet method in which both iodide and bromide are simultaneously introduced into a reactor vessel together with a silver salt, the distribution of silver iodide in the grains to be formed may be controlled. For instance, silver iodide may uniformly be distributed throughout the grains formed, or alternatively, if addition of bromide is reduced or stopped in the course of the formation of the grains while addition of iodide only is continued, a silver iodide shell or silver iodide-rich silver iodobromide shell may be formed on the outer surface (shell) of the grains. JP-A-58-113927 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") corresponding to U.S. Patent 4,434,226 mentions a silver halide emulsion wherein tabular silver iodobromide grains having a thickness of less than 0.5 µm, a diameter of 0.6 µm or more and a mean aspect ratio of 8/1 or more account for at least 50% of the total projected area of the grains therein, in which the tabular silver iodobromide grains have first and second parallel main surface planes which face each other and a center region extending between the two main planes. The silver iodide content in the center region is lower than that in at least one region which also extends between the two main planes and which is crosswise displaced. JP-A-59-99433 mentions a silver halide emulsion containing silver halide grains, in which tabular silver halide grains having an aspect ratio of 5 or more account for 10% (by number) of the silver halide grains. The grains contain silver iodide in the inside site by 80 mol% of the silver amount of the total grains from the center of the grains in the major axis or minor axis direction of the grains (inside iodine-rich phase). The mean iodine content in the inside iodine-rich phase is 5 times or more of the mean iodine content in the silver halide existing outside this phase, and the silver amount in the inside iodine-rich phase is 50 mol% or less of the silver amount of the whole grain. JP-60-147727 mentions a silver halide photographic emulsion containing multi-layered silver halide grains having an aspect ratio of 5 or less, in which the difference in the mean iodine content between the adjacent two layers, each having a uniform iodine distribution, is 10 mol% or less, and the total silver iodide content in the multi-layered silver halide grains is 10 mol% or less. In accordance with the techniques referred to in the above patent applications, the silver iodide content is varied in different sites in one grain (especially the content is varied between the inside and the outside of the grain), so as to obtain a better photographic characteristic. On the other hand, M.A. King, M.H. Lorretto, T.J. Maternaghan and F.J. Berry, The Investigation of Iodide Distribution by Analytical Electron Microscopy (Progress in Basic Principles of Imaging Systems, International Congress of Photographic Sicence, Köln, 1986) reported the result of topography of the silver iodide content in core/shell tabular silver iodide content in core/shell tabular silver iodobromide grains which comprise a tabular silver bromide grain core and an 11% silver iodide-containing silver iodobromide shell formed on the core, as measured by analytical electron microscopy. The result indicates that the silver iodide content in the silver iodobromide phase in the shell is higher in the center part (inner part) than in the peripheral part (outer part). That is, although the grains were grown so that the iodine content in the shell could be 11 mol% (uniform silver iodide phase) in the formation of the shells over the cores, the result was not so. JP-A-60-14331 mentions a silver halide photographic emulsion containing silver halide grains with a distinct layered structure, in which the grains are composed of a core part containing from 10 to 45 mol% of silver iodide and a shell part having 5 mol% or less silver iodide and have a mean silver iodide content of 7 mol% or more. JP-A-61-245151 corresponding to EP 202784A mentions a silver halide emulsion containing silver halide grains having a multi-layered structure with a different silver iodide content in the respective layers, in which the silver iodide content in the outermost layer is 10 mol% or less, a silver iodide-rich layer having a higher silver iodide content than the outermost layer by 6 mol% or more is provided inside the outermost layer, and a interlayer having an intermediate silver iodide content is provided between the outermost layer and the silver iodide-rich layer. In accordance with the techniques as referred to in the above patent applications, the silver iodide content is varied in different sites in one grain (especially the content is varied between the inside and the outside of one grain), so as to obtain a better photographic characteristic. On the other hand, in 1941's Annual Meeting of SPSE Y.T. Tan and R.C. Baetzold presented a presumption that iodine in silver iodobromide crystal grains would form a cluster based on a calculation of the energy state of silver halide. The distribution of silver iodide in the above-mentioned tabular silver iodobromide grains is derived from the variation of the silver iodide content in different sites of a unit of at least from 300 to 1000 Å or more in the grains. However, more microscopic and non-uniform silver iodide distribution is confirmed in silver iodobromide crystals, as so presumed by Tan and Baetzold. In accordance with the present invention, there is provided silver halide grains having a completely uniform silver iodobromide phase, and the grains are those in which the above-mentiond microscopic silver iodide distribution is completely uniform. The microscopic silver iodide distribution in silver iodobromide grains can be observed by a cooling type transmission electron microscope. The silver halide grains where the silver iodide distribution is completely uniform, which are provided by the present invention, could not be obtained by any conventional technical means. SUMMARY OF THE INVENTION The object of the present invention is to provide a negative type silver halide emulsion which has a high sensitivity with less fog, which has improved graininess and sharpness-covering power and which is excellent in storability and pressure-resistance. The object of the present invention has been met by a silver halide photographic emulsion comprising a dispersion medium and silver halide grains, which is characterized in that the silver halide grains include a silver halide phase containing 3 mol% or more of silver iodide, the microscopic non-uniformity caused by the silver iodide being such that the silver halide grains exhibit at most two lines at an interval of 0.2 µm in the direction perpendicular to the lines when observed by a transmission electron microscope, and the emulsion contains at least 60 wt.% of the said silver halide grains. The present invention further provides a process for preparing the above silver halide photographic emulsion comprising the steps of (1) mixing an aqueous solution of a water-soluble silver salt and an aqueous solution of at least one water-soluble halide in a mixer disposed outside a reaction vessel and (2) charging an aqueous solution of protective colloid into the mixer at a concentration of at least 1 wt.% in at least one of the following methods: (a) singly; (b) in the aqueous solution of the water-soluble silver salt; and (c) in the aqueous solution of the water-soluble halide, for preparing silver halide seed grains and (3) mixing the silver halide seed grains with fine iodide-containing silver halide grains in a reactor vessel rather than adding separate aqueous halide and silver salt solutions thereto. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a photograph taken by a transmission electron microscope at a magnification of 50,000 times. Fig. 1 shows the crystal structure of conventional tabular silver halide grains where the iodine distribution in the silver iodobromide phase is not completely uniform. Fig. 2 is an outline to typically show one embodiment of the process of preparing the emulsion of the present invention, in which fine silver halide grains are fed from the mixer vessel as provided outside the reactor vessel. In Figs. 3, (2-B), (2-C) and (2-D) photographs taken by a transmission electron microscope at a magnification of 50,000 times. These figures show the crystal structure of the tabular silver halide grains in the emulsion prepared in Example 2. In Figs. 3, (2-B), (2-C) and (2-D) are specific tabular silver halide grains in Emulsions (2-B), (2-C) and (2-D), respectively, prepared in Example 2. DETAILED DESCRIPTION OF THE INVENTION As one embodiment of the silver halide emulsion grains having a completely uniform silver iodide distribution of the present invention, tabular silver iodobromide grains having a silver iodobromide phase will be mentioned hereunder. Specifically, tabular silver iodobromide grains having a silver iodobromide phase with a completely uniform silver iodide distribution of the present invention will be mentioned hereunder. The "completely uniform silver iodide distribution" as herein referred to means a "more microscopic distribution", being different from the silver iodide distribution as heretofore been discussed for conventional silver halide grains. As a means of measuring the silver iodide distribution in silver iodobromide grains, an analytical electron microscope is frequently used. For example, King et al ( The Investigation of Iodide Distribution by Analytical Electron Microcopy , referred to above) mention that "the diameter of the electron ray spot to be irradiated onto the surface of a sample is to be actually about 30 nm (300 Å) or so since the electron ray irradiated is broadened because of the elastic scattering of electrons, although the size of the electron ray-irradiating probe is 5 nm (50 Å). Accordingly, it is impossible to measure a more fine and accurate silver iodide distribution than the said electron ray sot diameter by the above method. JP-A-58-113927 also uses the same means to measure the silver iodide distribution, in which the size of the electron ray spot used is 0.2 µm In accordance with the above measuring methods, therefore, it is impossible to clarify a more microscopic silver iodide distribution (especially, a variation of the distribution sites on the order of 10 nm (100 Å) or less) in the silver halide grains. The microscopic silver iodide distribution may be observed by the direct method with a transmission electron microscope at a low temperature, as described in J.F. Hamilton, Photographic Science and Engineering , Vol. 11, 1967, page 57 and T. Shozawa, Journal of Japan Photographic Association , Vol. 35, No. 4, 1972, page 213. Briefly, silver halide grains as taken out under a safelight so that the grains are not printed out and are set on a mesh for electron microscopic observation. The grains are observed by transmission electron microscopy under the condition that the sample is cooled with a liquid nitrogen or liquid helium so that the sample is not damaged (or printed out) by electron rays. The accelerated voltage of the electron microscope to be used in the method is better to be higher so as to obtain a more sharp microscopic image. Concretely, the voltage is preferably 200 KV for grains having a thickness of up to 0.25 µm, and it is preferably 1000 KV for grains having a thickness larger than 0.25 µm. If the accelerated voltage becomes higher, the damage of the grains by the irradiated electron rays would become larger. Accordingly, the sample is desired to be cooled with a liquid helium rather than with a liquid nitrogen. The photographing magnification is generally from 20,000 times to 40,000 times, although it may properly be varied in accordance with the grain size of the grains to be observed. For instance, when tabular silver iodobromide grains are photographed by transmission electron microscopy, extremely fine annual ring-like stripe patterns are observed in the portion of the silver iodobromide phase. One example of the pattern is shown in Fig. 1. The tabular grains shown in Fig. 1 are tabular core/shell grains prepared by forming a silver iodobromide shell (silver iodide: 10 mol%) around a tabular silver bromide grain core. The structure of the grains may distinctly be observed by the transmission electron microscopic photograph. That is, the core part is silver bromide and is naturally uniform, which is therefore seen as only a uniform flat image. On the other hand, extremely fine regular ring-like stripe patterns are clearly confirmed in the silver iodobromide phase. It is noted that the interval between the respective stripes in the pattern is extremely fine and small or is or the order of 100 Å or less and the stripes are extremely microscopically non-uniform. The extremely fine stripe patterns indicates the non-uniformity of the silver iodide distribution in the grains, which may be clarified by various methods. More directly, when the tabular grains are annealed under the condition that the iodide ion may move in the silver halide crystals (for example, at 250°C for 3 hours), the stripe patterns are lost. From this fact, non-uniformity may be concluded. The regular ring-like stripe patterns showing the non-uniformity of the silver iodide distribution in the tabular silver iodobromide emulsion grains as hereinbefore mentioned are also distinctly observed in the transmission electron microscopic photograph, attached to JP-A-58-113927 and in the transmission microscopic photograph in the aforesaid King et al's study. From these facts, the conventional silver iodobromide grains prepared to have a substantially uniform silver iodide distribution had, in fact, an extremely microscopically non-uniform silver iodide distribution, as opposed to the intended object of preparing grains with uniform silver iodide distribution. Despite this situation, a technique capable of unifying the silver iodide distribution could not be found until the present invention, and a process of preparing silver iodobromide grains with microscopically uniform silver iodide distribution could also not be found until the present invention. The tabular silver halide grains of the present invention having a "completely uniform silver iodide distribution", which have hereinbefore mentioned, can be distinctly differentiated from conventional silver halide grains by observing the transmission image of the grains with a cooling type transmission electron microscope. That is, the silver iodide-containing tabular silver halide grains of the present invention have at most two, preferably one, more preferably no, microscopic lines, which are caused by the microscopic non-uniformity of silver iodide, if any, at an interval of 0.2 µm. The lines constituting the regular ring-like stripe patterns show the microscopic non-uniformity of silver iodide occurring at a right angle to the direction of growth of the grain. As a result, the lines distribute in the form of concentric circles from the center of the grain. For instance, in the case of the tabular grain shown in Fig. 1, the lines constituting the regular ring-like stripe patterns, which show the non-uniformity of the silver iodide, are at a right angle to the direction of the growth of the tabular grain, so that the lines are parallel to the lines is towards the center of the grain, so that the regular ring-like stripe patterns formed of the lines are distributed in the form of concentric circles around the center of the grain. Needless to say, if the silver iodide content is rapidly varied during the growth of the grains, the boundary line caused by such rapid variation would be observed as a line which is similar to the above-mentioned lines for the regular ring-like stripe patterns by the above-mentioned observation method. However, the line caused by the rapid variation of the silver iodide content is a single line, which therefore may be distinctly differentiated from the plural lines caused by the microscopic non-uniformity of silver iodide distribution in the grain. Further, the line derived from the rapid variation of the silver iodide content may apparently be confirmed by measurement of the silver iodide content in both sides separated by the line by the above-mentioned analytical electron microscope. The lines caused by the variation of the silver iodide content is quite different from the lines caused by the microscopic non-uniformity of silver iodide distribution as herein referred to, and the former shows "macroscopic silver iodide distribution". When the silver iodide content is substantially continuously varied in the course of the growth of the grains, the lines showing the variation of the above-mentioned macroscopic silver iodide content are not observed since the silver iodide content does not rapidly vary. Accordingly, if the grain has at least 3 or more lines at an interval of 0.2 µm, these lines mean the existence of microscopic non-uniform silver iodide content in the grain formed. The silver halide grains of the present invention having a completely uniform silver iodide distribution are those having at most 2, preferably one, more preferably no, lines showing the microscopic silver iodide distribution at an interval of 0.2 µm in the direction perpendicular to the lines, as the transmission image of the grain obtained by the use of a cooling-type transmission electron microscope. The emulsion emulsion of the present invention contains such silver halide grains in an amount of at least 60%, preferably at least 80%, more preferably at least 90% of the total silver halide grains therein. Tabular silver halide grains which have heretofore been called "tabular silver halide grains containing a uniform silver iodide" were prepared by merely adding a silver nitrate and a halide mixture having a determined composition (or having a determined iodide content) to a reactor vessel by double jet method for growth of the grains in the reactor vessel. Therefore, the microscopic silver iodide distribution in the thus prepared grains is not uniform although the macroscopic silver iodide distribution therein may surely be uniform. Accordingly, such grains are called "tabular grains having a determined halogen composition in accordance with the present invention and these are distinctly differentiated from the "completely uniform tabular grains" of the present invention. Next, the method of preparing the tabular silver halide grains of the present invention will be mentioned hereunder. The method of preparing the grains of the present invention relates to the growth of tabular grains, and any and every conventional technique may be employed for nucleation of the grains. Briefly, nucleation of the tabular silver halide grains of the present invention is composed of two processes of nucleation of tabular grains and ripening of the resulting nuclei. Precisely, as mentioned in JP-A-61-299155, nucleation of tabular grains is accompanied by formation of other fine grains(especially octahedral grains and simplex twin plane grains), and after subsequent ripening of the resulting nuclei, other grains than tabular grain nuclei are lost so that only nuclei which are to be tabular grains may thereby be obtained. Accordingly, for formation of the tabular grains, the nucleation process includes two processes of generation of tabular nuclei and ripening of the resulting nuclei. The nucleation step is outside the technical scope of the present invention and, therefore, in the transmission image of the finally obtained tabular grains as photographed with a cooling type transmission electron microscope, the center part of the grain indicating the nucleus thereof is outside the subject matter of the present invention. After completion of nucleation, a different layer may be added to the resulting nucleus and grown thereon, whereby the boundary line between the nucleus and the grown phase may apparently be observed in the transmission image of the grain and the position of the nucleus in the grain may be confirmed from the boundary line. In particular, when the nucleus and the grown phase differ in the halide composition, the boundary line may be confirmed more clearly. The composition of the tabular silver halide grains of the present invention which have a completely uniform silver iodide distribution may be any one of silver iodobromide, silver iodochloride or silver iodochlorobromide, but it is preferably silver iodobromide or silver iodochlorobromide. Regarding the position of the silver iodide-containing phase in the grain, the phase may be in the center part of the tabular grain or may be throughout the whole grain or may also be in the peripheral part of the grain. The grain may have one or more silver iodide-containing phases. In general, the silver iodide-containing phase frequently forms a cyclic structure in the grain because of the mechanism of the growth thereof, but this may be in a particular site in the grain. For instance, the silver iodobromide phase may be formed only in the edges of the tabular grain or only in the corner parts thereof, by utilizing the difference in the properties between the edge and corner of the tabular grain. In addition, if a shell is formed outside therefrom, tabular silver halide grains which have silver iodide in a particular point and which do not have a cyclic structure may be formed. As examples, growth of the grains of the present invention may be conducted in accordance with the processes mentioned below. In the case of formation of silver iodochlorobromide grains, silver chloride may be added to the above-mentioned procedures, whereupon the silver chloride-containing layer may be any one of the first coating layer, the second coating layer or the third coating layer. In the present invention, the proportion of the uniform AgBrI to be in one tabular grain is preferably from 5 to 95 mol%, more preferably 30 to 95 mol%, and most preferably 50 to 95 mol%. The silver iodide content in the silver iodobromide phase in the emulsion grains of the present invention is from 3 to 45 mol%, preferably from 5 to 35 mol%. If the silver iodide content is less than 3 mol%, the existence of microscopically non-uniform silver iodide, if any, in the grain would cause only a substantially negligible width of the silver iodide distribution, which therefore is not so inconvenient. For example, when silver halide grains having a silver iodide-containing silver halide phase as the outermost layer are chemically sensitized and the silver iodide content in the outermost layer is less than 3 mol%, the matter whether or not the silver iodide distribution of the said silver halide phase is "completely uniform" is not related so much to the intended sensitivity and the degree of fog of a grain-containing emulsion. However, if the silver iodide content in the silver halide phase of the said silver iodide-containing outermost layer is 3 mol% or more, especially 5 mol% or more, the conventional grains having a non-uniform silver iodide distribution can not be conveniently chemically sensitized, or that is, the sensitivity attainable by chemical sensitization is extremely low and the degree of fog of the resulting emulsion is noticeably high. This means that the conventional grains having a silver halide phase with a "determined silver iodide composition" as the outermost layer are inhibited to be chemically sensitized, which is as mentioned hereinbefore. Accordingly, the conventional silver iodide-containing grains can not fully display all of the advantages of the silver iodide content in the grains, so that an emulsion containing such grains may have high sensitivity, low fog, good graininess and high sharpness, which could not heretofore be attained by the prior art. The reason why the grains having a non-uniform silver iodide distribution on the surface thereof is inhibited to be chemically sensitized while the grains having a completely uniform silver iodide distribution on the surface thereof is not is believed to be as follows: The lattice constant in the surface of the grain crystal is not constantly determined in the former non-uniform silver iodide distribution-having grains so that the composition of the chemically sensitized nuclei to be formed over the surface as well as the size of the nuclei would be non-uniform. Accordingly, an optimum chemical sensitization condition could not be obtained in this case. On the other hand, in the latter grains with a completely uniform silver iodide distribution-having surface, the composition and the size of the chemically sensitized nuclei can be uniform so that the grains can be optimally chemically sensitized. However, further investigation is necessary on this aspect. On the other hand, in the case of the grains having a silver iodide-containing silver halide phase in the inside of the grain, where the outermost layer has a low silver iodide content or has no silver iodide, it is expected that the band structure would be bent at the interface between both phases. Therefore, it is believed that positive holes, generated by light absorption because of the bending, are directed to the inside of the grains so that the charge separation of electrons from the positive holes are accelerated and the silver iodide in the inside of the grains captures the positive holes so as to inhibit re-bonding of the positive holes with electrons, whereby the sensitivity of the emulsion is elevated. It has been found that when the silver iodide distribution in the inside of the grains is completely uniform, the sensitivity attainable by chemical sensitization is high, but when the silver iodide distribution therein is non-uniform, the sensitivity attainable thereby is low. This is a surprising effect. Although the reason is not clarified as yet, it is expected that the completely uniform silver iodide distribution-having grains have a uniform positive hole-capturing capacity in the inside of the grains, while the non-uniform silver iodide distribution-having grains have a non-uniform positive hole-capturing capacity, so that they both remarkably differ from each other in the electron-positive hole re-bonding preventing effect. Also in this case, if the silver iodide content in the inside of the grains is less than 3 mol%, the attainable sensitivity does not substantially vary, irrespective of whether the silver iodide distribution of the grains is completely uniform or non-uniform, because of the same reason as mentioned above. However, when the silver iodide content is 3 mol% or more, especially 5 mol% or more, the sensitivity of the completely uniform silver iodide distribution-having grains is apparently higher than non-uniform grains. The total silver iodide content in the emulsion grains of the present invention is 2 mol% or more, and more effectively it is 4 mol% or more. Further preferably, it is 5 mol% or more. The size of the completely uniform silver iodide distribution-having tabular silver halide grains of the present invention is not specifically limited, but the size is preferably 0.5 µm or more as the mean projected area-corresponding diameter, and more effectively, it is 1.0 µm or more, especially 1.5 µm or more. The size distribution of the tabular silver halide grains in the emulsion of the present invention may be either monodispersed or polydispersed, but it is preferably monodispersed with respect to the shape and the grain size of the grains. Precisely, it is preferred that tabular silver halide grains having two parallel hexagonal outer surfaces where the ratio of the maximum side to the minimum side is 2 or less in one haxagonal plane account for 70% or more of the total projected area of all the grains in the silver halide emulsion of the present invention, like the emulsion mentioned in JP-A-61-299155. Further, the hexagonal tabular silver halide grains are more preferably monodispersed emulsion" as herein referred to means that the fluctuation coefficient of the grain size in the emulsion is 20% or less, preferably 15% or less. Next, the method for preparation of the tabular silver halide grains of the present invention will be mentioned hereunder. The tabular grains for use in the present invention are grains composed of two parallel facing main (111) planes and having a mean aspect ratio of 2 or more, preferably from 3 to 20, more preferably form 3 to 15. The grain size of the grains is 0.4 µm or more, preferably from 0.4 to 4 µm Preferably, tabular silver halide grains having a mean aspect ratio larger than 2/1 occupy 50% or more of the total projected area of the silver halide grains. The "mean aspect ratio ( γ )" as referred to herein may be defined by the following formula (1), when the tabular silver halide grains are oriented on a plane surface so that the two facing main planes of the thus oriented grain are horizontal to the plane surface. In the formula, Di means a diameter of a circle having the same area as the projected area of the i'th silver halide grain; and t i means the thickness of the grain in the direction vertical to the two facing main planes. N means a number which is necessary and indispensable for giving the mean aspect ratio of the silver halide grains and, in general, the value of N mostly satisfies the following formula (2). N ≳ 600 (2) The above-mentioned formula (1) indicates that the mean aspect ratio ( γ ) may be a mean value of the aspect ratio γ i of the respective silver halide grains. However, when the silver halide grains substantially satisfy the following formula (3) or (4), the value ( γ ') which is defined by the following formula (5) is substantially same as the value ( γ ). t i ≃ t j (i ≠ j ; i , j ≦ N) (3) D i / t i ≃ D j / t j (i ≠ j ; i , j ≦ N) (4) Accordingly, the mean aspect ratio may be defined by the formula for γ ', provided that this may fall within the range of the accuracy acceptable in measurement of the grain size. Next, the method for preparation of the tabular silver halide grains of the present invention will be mentioned in detail hereunder. The method for preparation of the silver halide grains of the present invention comprises nucleation (formation of tabular nuclei and ripening thereof) and growth of grains. As the subject matter of the present invention is directed to the growth of grains as mentioned above, the nucleation may be conducted by any conventional methods. 1. Nucleation: As mentioned in JP-A-61-299155, an aqueous solution of water-soluble silver salt and an aqueous solution of alkali halide(s) are reacted in an aqueous solution containing a dispersion medium, while the pBr value in the reaction system is kept at 1.0 to 2.5, for nucleation. Tabular silver halide grains have one or more twin planes in the inside thereof, and the twin plane formation corresponds to a so-called nucleation process. Accordingly, the nucleation condition is determined by the frequency of formation of twin planes, which depends upon various super-saturation factors (temperature in nucleation, gelatin concentration, addition rate of the aqueous silver salt solution and aqueous alkali halide solution, Br⁻ concentration, number of stirring rotation, iodine content in the aqueous alkali halide solution to be added, amount of silver halide solvent, pH, salt concentration). These are illustrated in JP-A-61-238808. By control of the nucleation condition, the shape of the nuclei grains (number of the twin planes per one grain) and the number of grains (for determining the grain size after growth of the grains) may be varied. In particular, when the temperature in nucleation is adjusted to be from 15°C to 39°C and the gelatin concentration therein is adjusted to be from 0.05 to 1.6% by weight, fine nuclei grains with uniform grain size distribution can be formed. 2. Ripening: As mentioned in JP-A-61-299155, fine tabular grain nuclei are formed in nucleation and at the same time a number of other fine grains (especially octahedral grains or singlet twin plane grains) are also formed. Prior to the grain-growing step which will be mentioned below, it is necessary to eliminate other grains than tabular grain nuclei and to obtain only the grain nuclei which will be tabular grains. For this, the grain nuclei formed in the above-mentioned nucleation are ripened. As the concrete method, one may follow the method described in JP-A-61-299155. 3. Growth of the grains: After completion of nucleation, an aqueous solution of water-soluble silver salt and an aqueous solution of alkali halide(s) are added to the reactor vessel so as to grow the resulting nuclei without forming any new nuclei. In accordance with the conventional method, aqueous solutions of silver salt and halide(s) are added to the reactor vessel with efficiently stirring. In this case, when a silver halide of a single halogen composition (for example, silver bromide, silver chloride) is grown, the silver halide phase is quite uniform so that any microscopical non-uniformity is not admitted by observation with a transmission electron microscope. Naturally, any non-uniform growth (apart from dislocation) would not occur in principle in a single halide composition. Accordingly, the "non-uniformity" as referred to herein does not apply to the growth of pure silver bromide or pure silver chloride, irrespective of the condition of preparing the grains. However, in the case of growth of silver halides of multi-halide composition (so-called mixed crystals), the non-uniform growth in the halide composition is a serious problem. For example, non-uniform distribution of silver halide can distinctly be observed by a transmission electron microscope, as already mentioned hereinbefore. The grains for use in the present invention may be in the form of other various shapes than the aforesaid tabular grains. For example, the grains may have a regular crystalline form (normal crystal grains), such as cubic, octahedral, dodecahedral, tetradecahedral, tetracosahedral (tri-octahedral, tetra-hexahedral, rhomboid), or hexatetracontahedral crystals, or may also have an irregular crystalline form, such as spherical or potato-like crystals. The silver halide grains for use in the present invention may be other than tabular grains, and the method for preparing non-tabular grains for use in the present invention will briefly be mentioned below. 1. Nucleation: The silver halide grains which may be nuclei of the silver halide grains of the present invention may be prepared by the methods described in P. Glafkides, Chimie et Phisique Photographique (published by Paul Montel, 1967), G.F. Duffin, Photographic Emulsion Chemistry (published by The Focal Press, 1966) and V.L. Zelikman et al, Making and Coating Photographic Emulsion (published by The Focal Press, 1964). Briefly, the grains may be prepared by any of an acid method, a neutrallization method, an ammonia method. Also, as a method of reacting a soluble silver salt and soluble halide(s), a single jet method, a double jet method, or a combination thereof may be used. A so-called reverse mixing method capable of forming silver halide grains in the presence of excessive silver ions can also be employed. As one system of the double jet method, a so-called controlled double jet method of keeping a constant pAg in a liquid phase of forming silver halide grains can also be employed. According to the method, a silver halide emulsion containing silver halide grains having a regular crystal form and almost uniform grain sizes can be obtained. Two or more kinds of silver halide emulsions separately prepared can be blended for use in the present invention. In preparation of the silver halide grain nuclei for use in the present invention, it is preferred that the nuclei prepared have a uniform halogen composition. For preparation of silver halide grains where the core nucleus is silver iodobromide, a double jet method or controlled double jet method is preferably employed. Although varying in accordance with the reaction temperature and the kind of the silver halide solvent used, the pAg value in preparation of the silver halide nuclei for the present invention is preferably from 7 to 11. In preparation of the nuclei, use of silver halide solvents is preferred as the time for formation of silver halide grains may be shortened. For instance, generally well known silver halide solvents such as ammonia or thioethers may be used for this purpose. Regarding the shape of the silver halide nuclei, the nuclei may be spherical or may also be octahedral, cubic or tetradecahedral, or these may further be in a mixed system thereof. The nuclei may be polydispersed or monodispersed, but they are more preferably monodispersed. The "monodispersed nuclei" as herein referred to have the same meaning as mentioned above. In order that the silver halide grains may have a uniform grain size, a method of varying or properly controlling the adding speed of the silver nitrate or aqueous alkali halide solution in accordance with the growing speed of the silver halide grains formed, for example, as described in British Patent 1,535,016 and JP-B-48-36890 (the term "JP-B" as referred to herein means an "examined Japanese patent publication") and JP-B-52-16364, and a method of varying the concentration of the aqueous solutions to be added, for example, as described in US-A-4,242,445 and JP-A-55-158124 are preferably employed, so that the grains may rapidly be grown within a range not exceeding the critical persaturation degree for the reaction system. In accordance with these methods, re-nucleation hardly occurs and the individual silver halide grains can be uniformly coated for growing. Thus, these methods are also preferably used in the case where the coating layer, which will be mentioned hereunder, is to be introduced into the grain. In accordance with the above-mentioned nucleation method, an aqueous silver salt solution and an aqueous halide solution are added to a reactor vessel having a dispersion medium-containing aqueous solution therein with efficient stirring. Apart from this method, fine silver halide grains having a small grain size may be added to the reactor vessel, in place of adding the aqueous silver salt solution and the aqueous halide solution thereto, and the fine grains may optionally subsequently be ripened in the reactor vessel for nucleation. This will be mentioned hereunder for the method of growing the grain nuclei. The size of the fine silver halide grains to be added in the method is preferably 0.1 µm or less, more preferably 0.06 µm or less, especially preferably 0.03 µm or less. The method for preparation of the fine silver halide grains for use in the method will be mentioned in detail in the item of "growth of grains" to follow. The fine silver halide grains have an extremely high solubility as the grain size thereof is extremely fine. Thus, they are rapidly dissolved immediately after being added to the reactor vessel to be decomposed to the constituting silver ion and halide ion. Accordingly, they are deposited on a slight amount of the fine grains as introduced into the reactor vessel to form nuclei grains. In the nucleation method, silver halide solvents may be used, if desired, which will be mentioned hereinafter. The nucleation temperature is preferably 50°C or higher, more preferably 60°C or higher. The fine silver halide grains may be added to the reactor vessel all at once or they may be gradually and continuously added thereto. In the latter case of continuous addition, the flow rate of the grains to be added may be constant or this may be accelerated over the course of time. In the step of forming nuclei of the silver halide grains and in the step of physical ripening of the grains, a cadmium salt, a zinc salt, a lead salt, a thallium salt, an iridium salt or a complex salt thereof, a rhodium salt or a complex salt thereof, or an iron salt or a complex salt thereof may be incorporated into the reaction system. 2. Growth of grains: After completion of nucleation, an aqueous solution of a water-soluble silver salt and an aqueous solution of alkali halide(s) are added to the reactor vessel so as to grow the resulting nuclei without forming any new nuclei. In accordance with the conventional method, aqueous solutions of silver salt and halide(s) are added to the reactor vessel with efficient stirring. In this case, when a silver halide of a single halogen composition (for example, silver bromide, silver chloride) is grown, the silver halide phase is completely uniform so that any microscopical non-uniformity is not admitted by observation with a transmission electron microscope. Various investigations have heretofore been conducted in this technical field, so as to attain uniform growth of silver halide grains for use in photography. It is known that the growing speed of silver halide grains is noticeably influenced by the silver ion concentration and the halide concentration in the reaction solution and by the equilibrium solubility of the reactants in the reaction solution. Accordingly, it is believed that if the concentrations of the reactants in the reaction solution (silver ion concentration, halide ion concentration) are non-uniform, the growing speed would vary in accordance with the different concentrations and the grains in the reaction solution would thereby non-uniformly grow. In order to overcome such local variation of the concentrations in the reaction solution, the techniques illustrated in US-A-3,415,650, GB-B-1,323,464 and US-A-3,692,283 are known. In accordance with the known means, a hollow rotary mixer which has slits in the cylindrical wall (the inside of the mixer is filled with an aqueous colloid, and more preferably the mixer is divided into two rooms, i.e., upper and lower rooms, by a disc) is provided in a reactor vessel filled with an aqueous colloid so that the rotary shaft of the mixer may be vertical to the reactor vessel. An aqueous halide solution and an aqueous silver solution are fed into the mixer from the top and bottom open mouths through feeding ducts while the mixer is rapidly rotated so that the solutions are rapidly blended and reacted together (when the mixer has, the separating disc, the aqueous halide solution and the aqueous silver salt solution as fed into the two rooms are diluted with the aqueous colloid as filled in each room, and these are rapidly blended and reacted near the outlet slits of the mixer), whereby the silver halide grains formed by the reaction are expelled out into the aqueous colloid in the reactor vessel because of the centrifugal force formed by rotation of the mixer and the grains are grown in the colloid in the reactor vessel. However, the problem of the non-uniform silver iodide distribution in the grains formed can not be overcome at all by this method, and the grains formed are observed by a cooling type transmission electron microscope to have regular ring-like stripe patterns which indicate the non-uniform distribution of silver iodide in the grains. JP-B-55-10545 mentions a technique of improving the local distribution of the ion concentrations so as to prevent the non-uniform growth of grains. In accordance with this method, a mixer filled with an aqueous colloid is provided in the inside of a reactor vessel filled with an aqueous colloid is provided in the inside of a reactor vessel filled with an aqueous colloid, an aqueous halide solution and an aqueous silver halide solution are separately fed into the mixer through feeding ducts so that the reaction solutions are rapidly and vigorously stirred and blended by the lower stirring blades (turbine blades) as equipped in the mixer to form and grow silver halide grains. Immediately, the thus grown silver halide grains are expelled out from the mixer by the upper stirring blades (as provided above the lower stirring blades) to the aqueous colloid in the reactor vessel through the opening mouth as provided in the upper portion of the mixer. However, the problem of the non-uniform silver iodide distribution also can not be overcome even by this method, and the grains are distinctly observed to have regular ring-like stripe patterns therein which indicate the non-uniform silver iodide distribution in the grains formed. Further, JP-A-57-92523 also mentions the same technique of overcoming the problem. In accordance with this method, the problem of the non-uniform silver iodide distribution can also not be overcome. JP-A-62-99751 mentions a photographic element containing silver bromide and silver iodobromide tabular grains having a mean diameter range of from 0.4 to 0.55 µm and an aspect ratio of 8 or more, and JP-A-62-115435 mentions the same element with the same grains having a mean grain size of from 0.2 to 0.55 µm. In the example, there has been illustrated a technique of growing tabular silver iodobromide grains, in which an aqueous silver nitrate solution and an aqueous potassium bromide solution are added to the reactor vessel in the presence of a protective colloid (bone gelatin) by a double-jet method while a silver iodide (AgI) emulsion (mean grain size: about 0.05 µm, bone gelatin: 40 g/mol of Ag) is simultaneously fed thereinto so that tabular silver iodobromide grains may be grown. However, even this method can not overcome the problem of the non-uniform silver iodide distribution in the tabular grains formed, and distinct regular ring-like stripe patterns, which indicate the existence of the non-uniform silver iodide distribution in the grains, are observed. Under the circumstances, it is obvious that the conventional techniques which have heretofore been known and disclosed can not attain formation of tabular silver halide grains having a completely uniform silver iodide distribution. The present inventors earnestly studied and at last have found that, in order to overcome the problem of the non-uniform silver iodide distribution in the growth of iodide-containing tabular silver halide grains, the silver ion and the halide ion(s) (iodide ion, bromide ion, chloride ion) for forming the grains are not added to the reactor vessel in the form of the respective aqueous solutions but, rather, fine silver halide grains having the intended halide composition are fed into the reactor vessel and the grains are grown therein to the desired tabular grains, whereby no regular ring-like stripe patterns are formed in the tabular grains thus grown and the tabular grains have a completely uniform silver iodide distribution. The result could not be attained by any conventional methods, and the technique of the present invention is in fact unexpected. More precisely, the process of the present invention of forming iodide-containing tabular silver halide grains having a completely uniform silver iodide distribution includes the following methods. (1) A method of adding previously prepared silver iodide-containing fine grains in the form of an emulsion to a reactor vessel: An emulsion containing fine silver halide grains (silver iodobromide, silver chloroiodobromide, silver iodochloride) having the same silver iodide content as that in the intended tabular grains (to be finally obtained) is previously prepared, and the fine grains-containing emulsion only is fed into the reactor vessel without feeding either an aqueous solution of a water-soluble silver salt nor an aqueous solution of water-soluble halide(s) thereto, and the fine grains are grown to the intended tabular grains which are nuclei or cores in the reactor vessel. (2) A method of feeding fine silver halide grains prepared in a mixer vessel (equipped outside a reactor vessel) to the reactor vessel. As one preferred method of efficiently feeding fine grains to the outside the reactor vessel and an aqueous solution of water-soluble silver salt, an aqueous solution of water-soluble halide(s) and an aqueous protective colloid are fed into the mixer vessel and rapidly blended therein to form extremely fine silver halide grains therein. The resulting grains are immediately and directly fed into the reactor vessel, whereupon neither an aqueous solution of a water-soluble silver salt nor an aqueous solution of water-soluble halide(s) is fed into the reactor vessel like the case of the above-mentioned method (1). US-A-2,246,938 mentions a method of growing coarse grains in an emulsion by blending coarse grains on which nothing has been adsorbed and fine grains on which nothing has also been adsorbed or by gradually adding a fine grains-containing emulsion to a coarse grains-containing emulsion. In accordance with the method disclosed therein, however, the same silver iodobromide emulsion is divided into two parts, ammonia is added to one part to ripen and then the other part is blended therewith or gradually added thereto. Accordingly, this method is quite different from the process of the present invention for growing silver iodide-containing tabular grains. In addition, this U.S. Patent is silient on the limitation of the silver iodide content in the silver halide grains formed. The silver iodide content in the grains formed in the example is only 2.6 mol%. JP-A-57-23932 mentions a method of growing silver halide grains, in which a fine grains-containing emulsion as prepared in the presence of a growth inhibitor is washed with water by decantation and re-dispersed. The resulting emulsion is again re-dissolved, and the solution thus prepared is added to the emulsion containing fine grains to be grown, whereby the fine grains are grown after dissolution thereof. In accordance with the method disclosed, fine grains having a smaller grain size are advantageously be obtained, but the re-dissolution of the fine grains in the reactor vessel is interfered with by the growth inhibitor. In addition, JP-A-57-23932 is silent on the halogen composition of the fine grains. In the example thereof, fine grains of pure silver bromide only are illustrated. Accordingly, the invention of JP-A-57-23932 should be said quite different from the present invention which relates to growth of silver iodide-containing tabular grains. US-A-3,317,322 and 3,206,313 mention a method of forming a core/shell grains-containing silver halide emulsion having a high internal sensitivity, in which a silver halide grain emulsion containing chemically sensitized core grains having a mean grain size of at least 0.8 µm is blended with another silver halide grain emulsion containing not chemically sensitized silver halide grains having a mean grain size of 0.4 µm or less. The resulting mixture is ripened so a to form shells over the cores. However, the method disclosed relates to the preparation of internal latent image-forming type grains having a high internal sensitivity. In the examples of these U.S. Patents, the silver iodide content for shell formation is only 2 mol% or less. Accordingly, the inventions of these U.s. Patents are quite different from the present invention which relates to surface latent image-forming type tabular grains having a high silver iodide content (3 mol% or more). JP-A-58-113927 mentions (page 207) that "Silver, bromide and iodide may be introduced initially or during the growing stage of the grains in the form of fine silver halide grains as suspended in a dispersion medium. Concretely, silver bromide, silver iodide and/or silver iodobromide grains may be introduced for the purpose." However, it is quite silent on the technique of growing tabular grains by addition of only a fine grains-containing emulsion to a reactor vessel without feeding aqueous solutions of silver salt and halide(s) thereto. JP-A-62-124500 mentions an example of growing host grains in a rector vessel from previously prepared extremely fine grains (about 0.02 µm) as put in the reactor vessel. However, the fine grains used therein are silver bromide grains, and this reference relates to growth of normal crystalline grains. Accordingly, this is quite different from the present invention. Next, the respective methods will be explained in detail hereunder. Regarding Method (1): In accordance with method (1), tabular grains which are to be nuclei or cores are previously put in the reactor vessel and an emulsion containing previously prepared fine grains having a small grain size is added to the reactor vessel, whereby the fine grains are dissolved by Ostwald ripening and are deposited on the nuclei or cores existing in the reactor vessel so that the nuclei or cores are grown to the intended tabular grains. In this case, the halide composition of the previously prepared fine grains is to have the same silver iodide content as that in the intended tabular grains to be finally obtained, which is silver iodobromide, silver chloroiodobromide or silver iodochloride. Regarding the grain size of the fine grains, the mean diameter is preferably 0.1 µm or less, more preferably 0.06 µm or less. In the present invention, the dissolution speed of the fine grains is an important factor, and use of silver halide solvents is preferred so as to accelerate the speed. As examples of the silver halide solvents to be used for this purpose, there may be mentioned water-soluble bromides, water-soluble chlorides, thiocyanates, ammonia, thioethers and thioureas. For example, there are thiocyanates (such as those described in US-A-2,222,264,2,448,534, 3,320,069), ammonia, thioether compounds (such as those described in US-A-3,271,157, 3,574,628, 3,704,130, 4,297,439, 4,276,347), thione compounds (such as those described in JP-A-53-144319, 53-82408, 55-77737), amine compounds (such as those described in JP-A-54-100717), thiourea derivatives (such as those described in JP-A-55-2982), imidazoles (such as those described in JP-A-54-100717), substituted mercaptotetrazoles (such as those described in JP-A-57-202531). In accordance with the process of the present invention, the temperature of growing the tabular grains is 50°C or higher, preferably 60°C or higher, more preferably 70°C or higher. For growing the nuclei crystals in the reactor vessel, the fine grains-containing emulsion may be added hereto all at a once, or alternatively, the emulsion may be divided into parts which are added one by one to the reactor vessel. Preferably, the fine grains-containing emulsion is fed into the reactor vessel at a constant flow rate, and more preferably, at an accelerated flow rate. In the latter case, the accelerating degree of the addition speed is determined in accordance with the concentration of the existing colloid, the solubility of the silver halide crystals, the size of the fine silver halide grains, the stirring degree in the reactor container, the size and concentration of the crystals as existing in each reaction stage, the hydrogen ion concentration (pH) and silver ion concentration (PAg) in the aqueous solution in the reactor container, and the size and distribution of the intended crystal grains to be finally obtained. Briefly, the addition speed to be accelerated may be determined on the basis of the related general experimental method. Regarding Method (2): In the method of growing silver halide crystal grains by the present invention, as mentioned above, fine silver halide crystal grains necessary for growing the silver halide crystal grains are added to the reactor vessel, without adding silver ion and halide ion(s) (containing iodide ion) in the form of respective aqueous solutions thereto (like the known conventional means) to cause Ostwald ripening because of the high solubility of the said fine grains, whereby the core nuclei existing in the reactor vessel are grown to the intended tabular grains. In this case, the reaction stage in the reaction system depends upon the speed of the dissolution of the fine grains as fed to release silver ion and halide ion(s) in the reactor vessel, but not the growing speed of the tabular grains to be finally obtained. In the case of method (1) where a previously prepared fine grains-containing emulsion is added to the reactor vessel, the fine grains to be added are desired to have a possibly minimum grain size. However, silver halide grains having a smaller grain size have a higher solubility, so that these would be extremely unstable to cause Ostwald ripening by themselves. As a result, the grain size of the grains would increase. T.H. James ( The Theory of the Photographic Process , 4th Ed.) refers to a Lippman emulsion as an example of fine grains and mentions that the mean grain size of the grains is 0.05 µm. Preparation of fine grains having a grain size of 0.05 µm or less is possible, but if obtained, the grains would be unstable and would easily undergo Ostwald ripening. As a result, the grain size of the resulting grains is increase. The Ostwald ripening may be prevented to some degree by adsorbing something to the fine grains, which, however would decrease the dissolution speed and is therefore contrary to the intended object of the present invention. In accordance with the present invention, the above problem can be overcome by the following three techniques. (A) The method has been mentioned as the aforesaid item (1), where fine grains are previously formed to give a fine grains-containing emulsion, the grains are re-dissolved, and the resulting fine grains-containing emulsion is added to a reactor vessel containing silver halide grains which are to be nuclei and also containing a silver halide solvent therein, so that the nuclei grains are grown in the vessel. In such a method, however, the extremely fine grains once formed undergo Ostwald ripening in the steps of grain formation, washing with water, re-dispersion and re-dissolution, so that the grain size of the resulting grains increases. In the system of the present invention according to the method described on page 40, lines 14 ff, as opposed to this, the mixer vessel is provided extremely near to the reactor vessel so that the residence time of the reaction solutions in the mixer vessel is shortened. Accordingly, the fine grains formed in the mixer vessel may immediately be introduced into the reactor vessel, whereby the Ostwald ripening is prevented. Specifically, the residence time (t) of the solutions as added to the mixer vessel is represented by the following formula: in which v means the volume of the reaction chamber in the mixer vessel (ml); a is the amount of the silver nitrate solution added (ml/min); b is the amount of the halide solution added (ml/min); and c is the amount of the protective colloid solution added (ml/min). In the process of the present invention, (t) is 10 minutes or less, preferably 5 minutes or less, more preferably 20 seconds or less. Accordingly, the fine grains formed in the mixer vessel may directly and immediately be introduced into the reactor vessel without the grain size thereof increasing further. (B) Strong and efficient stirring is effected in the mixer vessel. T.H. James ( The Theory of the Photographic Process , page 93) mentions that "Another form in addition to Ostwald ripening is coalescence. In coalescence ripening, crystals which have been far remote from one another before this are directly contacted and fused together to give greater crystals so that the grain size of the thus fused grains rapidly varies thereby. Both Ostwald ripening and coalescence ripening occur not only after deposition but also during deposition." Coalescence ripening as referred to in the above reference easily occurs especially when the grain size is extremely small, and more particularly when stirring is insufficient. In an extreme case, coalescence ripening often causes formation of crude bulky grains. In accordance with the process of the present invention, since the closed type mixer vessel as shown in Fig. 2 is used, the stirring blades in the reactor chamber may be rotated at a high rotation speed. Accordingly, strong and highly efficient stirring and mixing can be effected by the process of the invention, although such could not be effected by the use of a conventional open type reactor vessel. (In the conventional open type reactor vessel, if the stirring blades are rotated at a high rotation speed, the reaction solution would be scattered because of the centrifugal force by the high speed rotation, and further the reaction solution would be foamed. Therefore, such high speed rotation is impracticable in the conventional open type reactor vessel.) Thus the above-mentioned coalescence ripening may be prevented in the process of the present invention. As a result, fine grains having an extremely small grain size can be obtained by the process of the present invention. Specifically, the rotation speed of the stirring blades in the process of the present invention is 1000 r.p.m. or more, preferably 2000 r.p.m. or more, especially preferably 3000 r.p.m. or more. (C) An aqueous protective colloid is injected into the mixer vessel. The above-mentioned coalescence ripening may noticeably be prevented by impartation of a protective colloid to the fine silver halide grains. In accordance with the process of the present invention, the aqueous protective colloid is added to the mixer vessel by the following means. (a) The aqueous protective colloid is singly injected into the mixer vessel by itself. The concentration of the protective colloid may be 1% by weight or more, preferably 2% by weight or more, and the flow rate thereof is at least 20%, preferably at least 50%, more preferably 100% or more, of the sum of the flow amounts of the aqueous silver nitrate solution and aqueous halide solution. (b) The protective colloid is incorporated into the aqueous halide solution. The concentration of the protective colloid is 1% by weight or more, preferably 2% by weight or more. (c) The protective colloid is incorporated into the aqueous silver nitrate solution. The concentration of the protective colloid is 1% by weight or more, preferably 2% by weight or more. When gelatin is used, silver gelatin is formed from silver ion and gelatin and this gives a silver colloid by photolysis and pyrolysis. Accordingly, the silver nitrate solution and the protective colloid it is better that be blended immediately before use. The above-mentioned methods (a) to (c) may be employed singly or in combination thereof. If desired, all three methods (a) to (c) may be employed simultaneously. As the protective colloid which is used in the process of the present invention, gelatin is generally used, but any other hydrophilic colloid may also be used. Specific examples are described in Research Disclosure , Vol. 176, Item 17643 (December, 1978), IX. For determination of the grain size of the grains thus obtained by the means (A) to (C), above the grains are put on a mesh and are directly observed with a transmission electron microscope with a magnification of from 20,000 to 40,000 times. The grain size of the fine grains for use in the present invention is 0.06 µm or less, preferably 0.03 µm or less, more preferably 0.01 µm or less. In accordance with the method of the present invention, extremely fine grains having a small grain size may directly be fed into the reactor vessel, whereupon the dissolution speed of the fine grains is high so that the growing speed of the tabular grains in the reactor vessel is advantageously high. In the method of the present invention, therefore, the use of silver halide solvents is no longer indispensable, but silver halide solvents may of course be used also in the method of the present invention, if desired, for the purpose of further accelerating the growing speed or for any other purposes. Regarding the silver halide solvents for use in the present invention, the description for the aforesaid Method (1) may be referred to. In the method of the present invention, the feeding speed of silver ion and halide ion(s) to the reactor vessel may freely be controlled. These may be fed thereto at a constant speed, or preferably the feeding speed may be accelerated. The means of accelerating the feeding speed is mentioned in JP-B-48-36890 and 52-16364. For the other matters, the description for the aforesaid Method (1) may be referred to. In accordance with the method of the present invention, the halogen composition of the growing grains may freely be controlled during the growing step of the grains, and for example, it is possible to maintain a constant silver iodide content, or to vary the silver iodide content at a certain point, during the step of growing the tabular grains. The reaction temperature in the mixer vessel may advantageously be 60°C or lower, preferably 50°C or lower, more preferably 40°C or lower. When the reaction temperature is 35°C or lower, a low molecular weight gelatin, a synthetic high molecular compound having a protective colloidal action on silver halide grains or other natural high molecular compounds than gelatin are preferably used as a binder, since general gelatin would easily solidify at such a low temperature. Examples of high molecular compounds having a protective colloidal action on silver halide grains which can be used in the present invention include the following compounds. (a) Polyacrylamide Polymers : Homopolymer of acrylamide; copolymers of polyacrylamide and imidated polyacrylamide described in US-A-2,541,474; copolymers of acrylamide and methacrylamide described in DE-C-1,202,132; partially aminated acrylamide polymers described in US-A-3,284,207; and acrylamide polymers described in JP-B-45-14031, US-A-3,713,834 and 3,746,548 and GB-B-788,343. (b) Aminopolymers : Aminopolymers described in US-A-3,345,346, 3,706,504, and 4,350,759 and DE-C-2,138,872; quaternary amine-containing polymers described in GB-B-1,413,125 and US-A-3,425,836; polymers having amino and carboxyl groups described in US-A-3,511,818; and polymers described in US-A-3,832,185. (c) Polymers containing Thioether Group: Thioether group-containing polymers described in US-A-3,615,624, 3,860,428 and 3,706,564. (d) Polyvinyl Alcohols : Homopolymers of vinyl alcohol; organic acid monoesters of polyvinyl alcohol described in US-A-3,000,741; maleic acid esters of polyvinyl alcohol described in US-A-3,236,653; add copolymers of polyvinyl alcohol and polyvinyl pyrrolidone described in US-A-3,479,189. (e) Acrylic Acid Polymers : Homopolymers of acrylic acid; amino group-containing acrylate polymers described in US-A-3,832,185 and 3,852,073; halogenated acrylate polymers described in US-A-;4,131,471; and cyanoalkylacrylate polymers described in US-A-4,120.727. (f) Hydroxyquinoline-containing Polymers : Hydroxyquinoline-containing polymers described in US-A-4,030,929 and 4,152,161. (g) Cellulose or Starch Derivatives : Cellulose or Starch Derivatives described in GB-B-542,704 and 551,659 and US-A-2,127,573, 2,311,086 and 2,322,085. (h) Acetals : Polyvinyl acetals described in US-A-2,358,836, 3,003,879 and 2,828,204 and GB-B-771,155. (i) Polyvinyl Pyrrolidones ; homopolymers of vinyl pyrrolidone; and copolymers of acrolein and pyrrolidone described in FR-B-2,031,396. (j) Polystyrenes : Polystyrylamine polymers described in U.S. Patent 4,315,071; and halogenated styrene polymers described in US-A-3,861,918. (k) Ternary Polymers : Ternary Copolymers of acrylamide/acrylic acid/vinyl imidazole described in JP-B-43-7561 and GE-C-2,012,095 and 2,012,970. (l) Others : Azaindene group-containing vinyl polymers described in JP-A-59-8604; polyalkyleneoxide derivatives described in US-A-2,976,150; polyvinylaminimide polymers described in US-A-4,022,623; polymers described in US-A-4,294,920 and 4,089,688; polyvinyl pyridines described in US-A-2,484,456; imidazole group-containing vinyl polymers described in US-A-3,520,857; triazole group-containing vinyl polymers described in JP-B-60-658; polyvinyl-2-methylimidazoles and copolymers of acrylamide/imidazole described in Journal of Japan Photographic Association , Vol. 29, No. 1, page 18; dextran; and water-soluble polyalkyleneaminotriazoles described in ... Vol. 45, page 43 (1950). Low molecular weight gelatins may also be used in the present invention. The mean molecular weight of gelatins for use in the present invention is preferably 30,000 or less, more preferably 10,000 or less. Low molecular weight gelatins which are used in the present invention can be prepared generally as mentioned below. A gelatin which is generally used and which has a mean molecular weight of 100,000 is dissolved in water and gelatin-decomposing enzyme (gelatinase) is added thereto to decompose the gelatin molecules with the enzyme. For the method, the descriptions in R.J. Cox, Photographic Gelatin II (published by Academic Press, London, 1976), pages 233,251 and pages 335 to 346 may be referred to. In the case of the method, low molecular weight gelatins having a relatively narrow molecular weight distribution can advantageously be obtained since the position of the bond to be cleaved by the enzyme is determined. In the method, the longer the enzyme-decomposing time, the lower the molecular weight of the decomposed gelatins. Alternatively, there is another method where a general gelatin is heated and hydrolyzed under a low pH (pH of from 1 to 3) or high pH (pH of from 10 to 12) atmosphere. Using the aforesaid synthetic protective colloids, natural protective colloids and low molecular weight gelatins, formation of fine silver halide grains is possible at a temperature of 40°C or lower or even at a temperature of 35°C or lower, whereby the problems in the use of general gelatin as a protective colloid can completely be overcome. Regarding the concentration of the protective colloid to be used, the concentration of the protective colloid to be added to the mixer vessel in Method (A) (where fine silver halide grains formed in a mixer vessel are directly introduced into a reactor vessel) is 0.2% by weight or more, preferably 1% by weight or more, more preferably 2% by weight or more. When the protective colloid is incorporated into aqueous silver halide solution and/or aqueous halide solution, the concentration thereof is 0.2% by weight or more, preferably 1% by weight ore more, more preferably 2% by weight or more. In method (B) (where fine silver halide grains previously prepared outside a reactor vessel are added to the reactor vessel), the concentration of the aqueous protective colloid solution in the reactor vessel in previous preparation of the fine silver halide grains is 0.2% by weight or more, preferably 1% by weight or more, more preferably 2% by weight or more. The emulsion of the present invention is generally spectrally sensitized. As spectral sensitizing dyes for use in the present invention, there are generally methine dyes, which include, for example, cyanine dyes, merocyanine dyes, complex cyanine dyes, complex merocyanine dyes, holopolar cyanine dyes, hemicyanine dyes, styryl dyes and hemioxonol dyes. To these dyes can be applied various nuclei which are usually utilized for cyanine dyes as basic heterocyclic nuclei. That is, such nuclei include pyrroline nuclei, oxazoline nuclei, thiazoline nuclei, pyrrole nuclei, oxazole nuclei, thiazole nuclei, selenazole nuclei, imidazole nuclei, tetrazole nuclei and pyridine nuclei; the nuclei obtained by fusing alicyclic hydrocarbon rings to these nuclei; and the nuclei obtained by fusing aromatic hydrocarbon rings to these nuclei, such as indolenine nuclei, benzindolenine nuclei, indole nuclei, benzoxazole nuclei, naphthoxazole nuclei, benzothiazole nuclei, naphthothiazole nuclei, benzoselenazole nuclei, benzimidazole nuclei and quinoline nuclei. Each of these nuclei may be substituted on the carbon atom of the dye. To merocyanine dyes or complex merocyanine dyes can be applied 5-membered or 6-membered heterocyclic nuclei such as pyrazolin-5-one nuclei, thiohydantoin nuclei, 2-thiooxazolidine-2,4-dione nuclei, thiazolidine-2,4-dione nuclei, rhodanine nuclei or thiobarbituric acid nuclei, as nuclei having a ketomethylene structure. The amount of the sensitizing dye to be added to the silver halide emulsion being prepared can not be indiscriminately determined, and depends upon the kind of the additives added to the emulsion or the amount of the silver halide therein. However, it may be almost the same as that to be used in the preparation of conventional emulsions by conventional methods. Concretely, the amount of the sensitizing dye to be added may be from 0.001 to 100 mmol, preferably from 0.01 to 10 mmol, per mol of silver halide. The sensitizing dye is added to the emulsion before or after chemical ripening thereof. In the preparation of the silver halide grains of the present invention, the sensitizing dye is most preferably added to the emulsion during chemical ripening or before chemical ripening (for example during formation of the grains or during physical ripening of the grains formed). The emulsion of the present invention may further contain, together with the sensitizing dye, a dye having no spectral sensitizing action by itself or a material which does not substantially absorb visible lights but shows supersensitizing action. For example, it may contain a nitrogen-containing heterocyclic group-substituted aminostyryl compound (for example, as described in US-A-2,933,390 and 3,635,721), an aromatic organic acid/formaldehyde condensation product (for example, as described in US-A-3,743,510), a cadmium salt or an azaindene compound for the purpose. The combinations described in US-A-3,615,613, 3,615,641, 3,617,295 and 3,635,721 are especially useful. The silver halide emulsion of the present invention is generally chemically sensitized. For chemical sensitization, for example, the method described in H. Frieser, Die Grundlagen der Photographischen Prozesse mit Silberhalogeniden (published by Akademische Verlagsgesellschaft, 1968), pages 675 to 734 may be employed. Precisely, a sulfur sensitization method using a sulfur-containing compound capable of reacting with active gelatin or silver (e.g., thiosulfates, thioureas, mercapto compounds, rhodanines); a reduction sensitization method using a reducing substance (e.g., stannous salts, amines, hydrazine derivatives, formamidinesulfinic acid, silane compounds); or a noble metal sensitization method using a noble metal compound (e.g., gold complexes, as well as complexes of metals belonging to Group VIII of the Periodic Table such as platinum, iridium or palladium) can be used individually or as a combination thereof. The photographic emulsion of the present invention can contain various compounds for the purpose of preventing of photographic materials and for the purpose of stabilizing the photographic property of the materials. For example, various compounds which are known as an anti-foggant or stabilizer can be added for the above purposes, which compounds include azoles, such as benzothiazolium salts, nitroindazoles, traizoles, benzotriazoles, benzimidazoles (especially nitro- or halogen-substituted derivatives); heterocyclic mercapto compounds, such as mercapthothiazoles, mercaptobenzothiazoles, mercaptobenzimidazoles, mercaptothiadiazoles, mercaptotetrazoles (especially 1-phenyl-5-mercaptotetrazole), mercaptopyrimidines; the heterocyclic mercapto compounds having a water-soluble group such as carboxyl group or sulfone group; thioketo compounds, such as oxazolinethione; azaindenes, such as tetrazaindenes (especially 4-hydroxy-substituted (1,3,3a,7)-tetrazaindenes); benzenethiosulfonic acids; and benzenesulfinic acids. The anti-foggant or stabilizer is generally added to the emulsion after chemical sensitization thereof, and more preferably, it is added thereto in the course of chemical ripening of the emulsion or at any time selected from prior to the beginning of chemical ripening of the emulsion. Specifically, it may be added to the emulsion at any time in the course of the formation of the silver halide grains of the emulsion, for example, during the course of addition of the silver salt solution, or after addition of the solution and before beginning of chemical ripening of the emulsion, or during the course of chemical ripening of the emulsion (preferably within the time of 50% from the beginning of chemical ripening, more preferably within the time of 20% therefrom). The emulsion of the present invention may be applied to any of mono-layered and multi-layered photographic materials having one or more layers in the form of any desired layer constitution. As one embodiment, the emulsion of the present invention may be applied to a silver halide multi-layer color photographic material, which has a multi-layered structure composed of laminates of binder and silver halide grains-containing layers, for the purpose of separately recording blue light, green light and red light, and the respective emulsion layers comprise two of a high sensitive layer and a low sensitive layer. As examples of particularly practicable layer constitutions, there are the following modifications. (1) BH/BL/GH/GL/RH/RL/S (2) BH/BM/BL/GH/GM/GL/RH/RM/RL/S (3) BH/BL/GH/RH/GL/RL/S (described in US-A-4,184,876) (4) BH/GH/RH/BL/GL/RL/S (described in Research Disclosure Item 22534 and JP-A-59-177551 and 59-177552) In the layer constitutions, B means a blue-sensitive layer, G means a green-sensitive layer, R means a red-sensitive layer, H means a highest sensitive layer, M means a middle sensitive layer, L means a low sensitive layer and S means a support. Other non-light sensitive layers such as a protective layer, filter layer, interlayer, anti-halation layer or subbing layer are not mentioned. Among the modifications, (1), (2) and (4) are preferred. In addition, the following modifications (described in JP-A-61-34541) are also preferred. (5) BH/BL/CL/GH/GL/RH/RL/S (6) BH/BL/GH/GL/CL/RH/RL/S In the layer constitutions (5) and (6), CL mans an interlayer effect-imparting layer, and the others have the same meanings as mentioned above. In the same color-sensitive layers, the high sensitive layer and the low sensitive layer may be arranged reversely. The silver halide emulsion of the present invention may be applied to color photographic materials as mentioned above, and it may further be applied to other mono-layered or multi-layered photographic materials having one or more emulsion layers, such as X-ray photographic materials, picture-taking black-and-white photographic materials, photographic materials for photomechanical process and printing photographic papers. Various additives to be added to the silver halide emulsion of the present invention, such as binder, chemical sensitizer, spectral sensitizer, stabilizer, gelatin hardening agent, surfactant, antistatic agent, polymer latex, mat agent, color coupler, ultraviolet absorbent, anti-fading agent and dye, as well as supports for the emulsion-having photographic materials, the coating method, the light-exposing method and the developing method are not specifically limited, and for example, the descriptions of Research Disclosure , Vol. 176, Item 17643 (RD-17643), ibid ., Vol. 187, Item 18716 (RD-18716) and ibid ., Vol. 225, Item 22534 (RD-22534) may be referred to. The related descriptions in these reference, are shown below. The color couplers for use in the present invention are preferably non-diffusible as having a ballast group or being polymerized. 2-Equivalent couplers in which the coupling active position is substituted by a coupling-releasing group are more preferred than 4-equivalent couplers where a hydrogen atom is in the coupling active position, because the amount of silver to be coated may be reduced. In addition, couplers capable of forming a colored dye with a pertinent diffusibility, colorless couplers, DIR couples capable of releasing a development inhibitor by a coupling reaction, or couplers capable of releasing a development accelerator by a coupling reaction may also be used. As the yellow couplers for use in the present invention, there are oil-protect type acylacetamide couplers as the typical examples. Specific examples thereof are described in US-A-2,407,210, 2,875,057 and 3,265,506. In the present invention, 2-equivalent yellow couplers are preferably used, and specific examples of these yellow couplers are the oxygen atom-releasing type yellow couplers described in US-A-3,408,194, 3,447,928, 3,933,501 and 4,022,620, and the nitrogen atom-releasing type yellow couplers described in JP-B-58-10739, US-A-4,401,752 and 4,326,024, RD-18053 (April, 1979), GB-B-1,425,020, DE-A-Nos. 2,219,917, 2,261,361, 2,329,587 and 2,433,812. Among these yellow couplers, α-pivaloylacetanilide couplers are excellent in fastness, in particular, light fastness of the colored dyes formed, while α-benzoylacetanilide couplers are excellent in coloring density. As the magenta couplers for use in the present invention, there are oil-protect type indazolone or cyanoacetyl couplers, preferably pyrazoloazole couplers such as 5-pyrazolones or pyrazolotriazoles. As the 5-pyrazolone couplers, those substituted by an arylamino or acylamino group at the 3-position thereof are preferred from the viewpoint of the hue and coloring density of the colored dyes formed. Specific examples of these couplers are described in US-A-2,311,082, 2,343,703, 2,600,788, 2,908,573, 3,062,653, 3,152,896 and 3,936,015. Also, as the releasable groups for the 2-equivalent 5-pyrazolone couplers, the nitrogen atom-releasing groups described in US-A-4,310,619and the arylthio groups described in US-A-4,351,897 are especially preferred. Furthermore, the 5-pyrazolone magenta couplers having a ballast group described in EP-B-73,636 give high coloring density. As the pyrazoloazole couplers, there may be mentioned the pyrazolobenzimidazoles described in US-A-3,061,432, preferably the pyraozolo[5,1-c][1,2,4]triazoles described in RD-24220 (June, 1984)and JP-A-60-33552, and the pyrazolopyrazoles described in RD-24230 (June, 184) and JP-A-60-43659. The imidazo[1,2-b]pyrazoles described in US-A-4,500,630 are preferred because of the small yellow side-absorption of the colored dye and the sufficient light-fastness thereof. In particular, the pyraozolo[1,5-b][1,2,4]triazoles described in US-A-4,540,654 are especially preferred. As the cyan couplers for use in the present invention, there are oil-protect type naphthol couplers. As typical examples of the couplers, there be mentioned the naphthol couplers described in US-A-2,474,293, preferably the oxygen atom-releasing 2-equivalent naphthol couplers described in US-A-4,052,212, 4,146,396, 4,228,233 and 4,296,200. Specific examples of phenol couplers which may be used in the present invention are described in US-A-2,369,929, 2,801,171, 2,772,162 and 2,895,826. Cyan couplers having high fastness to humidity and temperature are preferably used in the present invention and specific examples of these cyan couplers include the phenol cyan couplers having an alkyl group of 2 or more carbon atoms at the meta-position of the phenol nucleus described in US-A-3,772,002; the 2,5-diacylamino-substituted phenol cyan couplers described in US-A-2,772,162, 3,758,308, 4,126,396, 4,334,011 and 4,327,173, DE-A-. 3,329,729 and EP-B-121,365; and the phenol cyan couplers having a phenylureido group at the 2-position thereof and an acylamino group at the 5-positoin thereof described in US-A-3,446,622, 4,333,999, 4,451,559and 4,427,767. In addition, the naphthol cyan couplers having a sulfonamido or amido group at the 5-position of the naphthol nucleus described in JP-A-59-93605, 59-264277 and 59-268135 are also excellent in the fastness of the color image formed therefrom, and these may preferably be used in the present invention. In order to correct the unnecessary absorption in the short wavelength range by the dyes formed from magenta and cyan couplers, colored couplers are preferably used in picture-taking color negative photographic materials. As specific examples of colored couplers to be used for this purpose, there are the yellow-colored magenta couplers described in US-A-4,163,670 and JP-B-57-39413, and the magenta-colored cyan couplers described in US-A-4,004,929 and 4,138,258 and GB-B-1,146,368. In the present invention, by using couplers giving colored dyes having a proper diffusibility together with the aforesaid color couplers, the graininess of the color images formed can be improved. Specific examples of magenta couplers of this type are described in US-A-4,366,237 and GB-B-2,125,570, and specific examples of yellow, magenta and cyan couplers of this type are described in EP-B-96,570 and DE-A-. 3,234,533. The dye-forming couplers and the above-mentioned particular couplers for use in the present invention may form dimers or higher polymers. Typical examples of the polymerized dye-forming couplers are described in US-A-3,451,820 and 4,080,211. Also, specific examples of the polymerized magenta couplers are described in GB-B-2,102,173, US-A-4,367,282 and JP-A-60-75041 and 60-113596. So-called DIR couplers capable of releasing a development inhibitor along with coupling may also be used in the present invention. As examples of DIR couplers, there may be mentioned the couplers which release a heterocyclic mercapto-type development inhibitor, described in US-A-3,227,554; the couplers which release a benzotriazole derivative as a development inhibitor, described in JP-B-58-9942; the couplers which are so-called colorless DIR couplers, described in JP-B-51-16141; the couplers which release a nitrogen-containing heterocyclic development inhibitor via methylol decomposition after release of the inhibitor-containing group, described in JP-A-52-90932; the couplers which release a development inhibitor via an intramolecular nucleating reaction after release of the inhibitor-containing group, described in US-A-4,248,962 and JP-A-57-56837; the couplers which release a development inhibitor via an electron transfer in a conjugated system after release of the inhibitor-containing group, described JP-A-56-114946, 57-154234, 57-188035, 58098728, 58-209736, 58-209737, 58-209738, 58-209739 and 58-209740; the couplers which release a diffusive development inhibitor which deactivate a developer, described in JP-A-57-151944 and 58-217932; and the couplers which release a reactive compound which forms a development inhibitor or deactivates the development inhibitor by reaction in the film during development, described in JP-A-59-38263 and 59-39653. Among the above-mentioned DIR couplers, those which are especially preferably used in combination with the photographic materials of the present invention are developer-deactivating type DIR couplers, such as those described in JP-A-57-151944; the timing-type DIR couplers, such as those described in US-A-4,248,962 and JP-A-57-154234; and the reactive type DIR couplers, such as those described in JP-A-59-39653. Among them, in particular, the developer-deactivating type DIR couplers described in JP-A-57-151944 and 58-217932, JP-A-59-75474, 59-82214 and 59-90438, and the reactive type DIR couplers described in JP-A-59-39653 are especially preferred. The photographic materials of the present invention can contain compounds which may imagewise release a nucleating agent or a development accelerator or a precursor thereof (hereinafter referred to as "development accelerator or the like") in development. Typical examples of such compounds are described in GB-B-2,097,140 and 2,131,188. The compounds are so-called DAR couplers which release a development accelerator by a coupling reaction with the oxidation product of an aromatic primary amine developing agent. The development accelerator or the like to be released from such DAR couplers is preferred to have an absorbability to silver halides, and specific examples of such DAR couplers are described in JP-A-59-157638 and 59-170840. In particular, DAR couplers which release an N-acyl-substituted hydrazine compound having a monocyclic or condensed-heterocyclic absorbing group, from the coupling active position or the coupler via the sulfur atom or nitrogen atom are especially preferred, and specific examples of such couplers are described in JP-A-60-128446. As examples of high boiling point organic solvents to be used for dispersion of the above-mentioned color couplers, there may be mentioned phthalic acid esters (e.g., dibutyl phthalate, dicyclohexyl phthalate, di-2-ethylhexyl phthalate, decyl phthalate), phosphoric acid or phosphonic acid esters (e.g., triphenyl phosphate, tricresyl phosphate, 2-ethylhexyl-diphenyl phosphate, tricyclohexyl phosphate, tri-2-ethylhexyl phosphate, tridodecyl phospahte, tributoxyethyl phospahte, trichloropropyl phosphate, di-2-ethylhexylphenyl phosphonate), benzoic acid esters (e.g., 2-ethylhexyl benzoate, dodecyl benzoate, 2-ethylhexyl p-hydroxybenzoate), amides (e.g., diethyldodecanamide, N-tetradecylpyrrolidone), alcohols or phenols (e.g., isostearyl alcohol, 2,4-di-tert-amylphenol), aliphatic carboxylic acid esters (e.g., dioctyl azelate, glycerol tirbutyrate, isostearyl lactate, trioctyl citrate), aniline derivatives (e.g., N, N-dibutyl-2-butoxy-5-tert-octylaniline), hydrocarbons (e.g., paraffin, dodecylbenzene, diisopropylnaphthalene). As an auxiliary solvent, organic solvents having a boiling point of from about 30°C, preferably from about 50°C, to about 160°C are advantageously used, and specific examples of such organic solvents include ethyl acette, butyl acetate, ethyl propionate, methyl ethyl ketone, cyclohexanone, 2-ethoxyethyl acetate and dimethylformamide. Supports which may suitably be used in the present invention are described, for example, in RD-17643, page 28 and RD-18716, from page 647, right-hand column to page 648, left-hand column. As the binder for the silver halide emulsion of the present invention, gelatin is preferred. However, in addition to gelatin, other gelatin derivatives such as phthalated gelatin, as well as dextran, cellulose derivatives, polyvinyl acetate, polyacrylamide and polyvinyl alcohol may also be used. As the gelatin hardener, for example, active halogen compounds (e.g., 2,4-dichloro-6-hydroxy-1,3,5-triazine and sodium salt thereof) as well as active vinyl compounds (e.g., 1,3-bisvinylsulfonyl-2-propanol, 1,2-bis(vinylsulfonylacetamide)ethane or vinyl polymers having a vinylsulfonyl group in the side chain) are preferred, as these may rapidly harden hydrophilic colloids such as gelatin to give a stable photographic characteristic to the thus hardened emulsion. In addition, N-carbamoylpyridinium salts (e.g., 1-morpholinocarbonyl-3-pyridinio)methanesulfonate) and haloeamidinium salts (e.g., 1-(1-chloro-1-pyridinomethylene)phrrolidinium-2-naphthalene-sufonate) are also preferred, as having a high hardening speed. The color photographic materials containing the silver halide photographic emulsion of the present invention can be developed by conventional methods, for example, in accordance with the methods described in RD-17643, pages 28 to 29 and RD-18716, page 651, from left-band column to right-hand column. The color photographic materials containing the silver halide photographic emulsion of the present invention are, after being developed and bleach-fixed or fixed, generally rinsed with water or stabilized. In the rinsing step, two or more rinsing tanks are generally used in a countercurrent system, so as to economize the rinsing water to be used. Stabilization may be effected in place of rinsing in water, and one typical example is the multi-stage countercurrent stabilization system described in JP-A-57-8543. The color developer for use in development of the photographic materials of the present invention is preferably an aqueous alkaline solution consisting essentially of an aromatic primary amine developing agent. As the color developing agent for the developer, p-phenylenediamine compounds are preferably used, although aminophenol compounds are useful. Specific examples of the compounds include 3-methyl-4-amino-N,N-diethylaniline, 3-methyl-4-amino-N-ethyl-N-β-hydroxyethyl-aniline, 3-methyl-4-amino-N-ethyl-N-β-methyanesulfonamidoethyl-aniline, 3-methyl-4-amino-N-ethyl-N-β-methoxyehtylaniline and sulfates, hydrochlorides and p-toluenesulfonates thereof. Two or more of these compounds may be used in combination, in accordance with the object thereof. The color developer generally contains a pH buffer such as an alkali metal carbonates, borates or phosphates, and a development inhibitor or an antifoggant such as bromides, iodides, benzimidaozles, benzothiazoles or mercapto compounds. In addition, this may further contain, if desired, various kinds of preservatives, such as hydroxylamine, diethylhydroxylamine, sulfates, hydrazines, phenylsemicarbazides, triethanolamine, catecholsulfonic acids, triethylenediamine (1,4-diazabicyclo[2,2,2]-octanes); an organic solvent such as ethylene glycol or diethylene glycol; a development accelerator such as benzyl alcohol, polyethylene glycol, quaternary ammonium salts or amines; a dye-forming coupler; a competing coupler; a foggant such as sodium boronhydride; an auxiliary developing agent such as 1-phenyl-3-pyrazolidone; a tackifier; as well as various kinds of chelating agents such as aminopolycarboxylic acids, aminopolyphosphonic acids, alkylphosphonic acids or phosphonocarboxylic acids, e.g., ethylenediamine-tetraacetic acid, nitrilo-triacetic acid, diethylenetriamine-pentaacetic acid, cyclohexanediamine-tetraacetic acid, hydroxyethylimino-diacetic acid, 1-hydroxyethlidene-1,1-diphosphonic acid, nitrilo-N,N,N-trimethylenephosphonic acid, ethylenediamine-N, N,N',N'-tetramethylenephosphonic acid, ethylenediamine-di(o-hydroxyphenylacetic acid) and salts thereof. When reversal processing is carried out, the photographic materials are first subjected to black-and-white development and then to color development. The black-and-white developer to be used in the black-and-white development may contain known black-and-white developing agents, for example, dihydroxybenzenes such as hydroquinone, 3-pyraozlidones such as 1-phenyl-3-pyrazolidone or aminophenols such as N-methyl-p-aminophenol, singly or in combination thereof. The color developer and black-and-white developer generally have a pH value of from 9 to 12. The amount of the replenisher to the developer is, although depending upon the color photographic materials to be processed, generally 3 liters or less per m² of the material. By lowering the bromide ion concentration in the replenisher, the amount may be 500 ml or lower. When the amount of the replenisher to be added is lowered, it is desired to prevent the evaporation and aerial oxidation of the processing solution by reducing the contact surface area of the processing tank with air. In addition, the amount of the replenisher to be added may also be reduced by means of suppressing accumulation of bromide ion in the developer. After being color developed, the photographic emulsion layer is generally bleached. Beaching may be carried out simultaneously with fixation (bleach-fixation) or separately from the latter. In order to accelerate the photographic processing, bleaching may be followed by bleach-fixation. In addition, bleach-fixation in continuous two processing tanks, fixation prior to bleach-fixation or bleach-fixation followed by bleaching may also be applied to the photographic materials of the present invention, in accordance with the object thereof. As the bleaching agent can be used, for example, compounds of polyvalent metals such as iron (III), cobalt (III), chromium (VI) or copper (II), as well as peracids, quinones and nitro compounds. Specific examples of the bleaching agent include ferricyanides; bichromates; organic complexes of iron (III) or cobalt (III), for example, complexes with aminopolycarboxylic acids such as ethylenediamine-tetraacetic acid, diethylenetriamine-pentaacetic acid, cyclohexanediamine-tetraacetic acid, methylimino-diacetic acid, 1,3-diaminopropane-tetraacetic acid or glycolether-diamine-tetraacetic acid, as well as with citric acid, tartaric acid or malic acid; persulfates; bromates; permanganates; and nitrobenzenes. Among them, aminopolycarboxylic acid/iron (III) complexes such as ethylenediamine-tetraacetic acid/iron (III) complex as well as persulfates are preferred in view of the rapid processability thereof and of the prevention of the environmental pollution. The aminopolycarboxylic acid/iron (III) complexes are especially useful both in a bleaching solution and in a bleach fixing solution. The bleaching solution or bleach-fixing solution containing such aminopolycarboxylic acid/iron (III) complexes generally has a pH value of from 5.5 to 8, but the solution may have a lower pH value for rapid processing. The bleaching solution, bleach-fixing solution and the previous bath may contain a bleaching accelerating agent, if desired. Various bleaching accelerating agents are known, and examples of the agents which are advantageously used in the present invention include the mercapto group or disulfide group-containing compounds described in US-A-3,893,858, DE-C-1,290,812, JP-A-53-95630 and Research Disclosure, item 17129 (July, 1978); the thiazolidine derivatives described in JP-A-50-140129; the thiourea derivatives described in US-A-3,706,561; the iodides described in JP-A-58-16235; the polyoxyethylene compounds described in West German Patent 2,748,430; the polymine compounds described in JP-B-45-8836; and bromide ion. Among them, the mercapto group or disulfido group-having compounds are preferred because of the high accelerating effect thereof, and in particular, the compounds described in US-A-3,893,858, DE-C-1,290,812 and JP-A-53-95630 are especially preferred. In addition, the compounds described in US-A-4,552,834 are also added to photographic materials. When color photographic materials are bleach-fixed, the bleaching accelerating agents are especially effective. As the fixing agent, there are mentioned thiosulfates, thiocyanates, thioether compounds, thioureas and a large amount of iodides. Among them, thiosulfates are generally used, and in particular, ammonium thiosulfate is most widely used. As the preservative for the bleach-fixing solution, sulfites, biuslfites and carbonyl-bisulfite adducts are preferred. The silver halide color photographic materials are of the present invention generally rinsed in water and/or stabilized, after being desilvered. The amount of the water to be used in the rinsing step can be set in a broad range, in accordance with the characteristic of the photographic material being processed (for example, depending upon the raw material components, such as the coupler and so on) or the use of the material, as well as the temperature of the rinsing water, the number of the rinsing tanks (the number of the rinsing stages), the replenishment system of normal current or countercurrent and other various kinds of conditions. Among these conditions, the relation between the number of the rinsing tanks and the amount of the rinsing water in a multi-stage countercurrent rinsing system can be obtained by the method described in Journal of the Society of Motion Picture and Television Engineers , Vol. 64, pages 248to 253 ( May, 1955). According to the multi-stage countercurrent system described in the above-reference, the amount of the rinsing water to be used can be reduced noticeably, but because of the prolongation of the residence time of the water in the rinsing tank, bacteria would propagate in the tank so that the floating substances generated by the propagation of bacteria would adhere to the surface of the material as it was processed. Accordingly, the above system would often have a problem. In the practice of processing the photographic materials of the present invention, the method of reducing calcium and magneisum ions, which is described in JP-A-61-131632, can extremely effectively be used for overcoming this problem. In addition, the isothiazolone compounds and thiabendazoles described in JP-A-57-8542; chlorine-containing bactericides such as chlorinated sodium isocyanurates; and benzotriazoles and other bactericides described in H. Horiguchi, Chemistry of Bactericidal and Fungicidal Agents , and Bactericidal and Fungicidal Techniques to Microorganisms , edited by Association of Sanitary Technique, Japan, and Encyclopedia of Bactericidal and Fungicidal Agents , edited by NIppon Bactericide and Fungicide Association, can also be used. The pH value of the rinsing water to be used for processing the photographic materials of the present invention is from 4 to 9, preferably from 5 to 8. The temperature of the rinsing water and the rinsing time can also be set variously in accordance with the characteristics of the photographic material being processed as well as the use thereof, and in general, the temperature is from 15 to 45°C and the time is from 20 seconds to 10 minutes, and preferably the temperature is from 25 to 40°C and the time is from 30 seconds to 5 minutes. Alternatively, the photographic materials of the present invention may also be processed directly with a stabilizing solution in place of being rinsed with water. For the stabilization, any known methods, for example as described in JP-A-57-8543, 58-14834 and 60-220345, can be employed. In addition, the material can also be stabilized, following the rinsing step. As one example thereof, there may be mentioned a stabilizing bath containing formaldehyde and a surfactant, which is used as a final bath for color photographic materials. The stabilizing bath may also contain various chelating agents and fungicides. The overflow from the rinsing and/or stabilizing solutions because of addition of replenishers thereto may be re-used in the other steps such as the previous desilvering step. The silver halide photographic materials of the present invention can contain a color developing agent for the purpose of simplifying and accelerating the processing of the materials. For incorporation of color developing agents into the photogrpahic materials, various precursors of the agents are preferably used. For example, there are mentioned the indoaniline compounds described in US-A-3,342, 597, the Schiff base compounds described in US-A-3,342,599 and Research Disclosrue Items 14850 and 15159, the aldole compounds described in Research disclosure Item 13924, the metal complexes described in US-A-3,719,492 and the urethane compounds described in JP-A-53-135628, as the precursors. The silver halide color photographic materials of the present invention can contain various kinds of 1-phenyl-3-pyrazolidones, if desired, for the purpose of accelerating the color developability thereof. Specific examples of these compounds are described in JP-A-56-64339, 57-144547 and 58-115438. The processing solutions for the photographic materials of the invention are used at 10°C to 50°C. In general, a processing temperature of from 33°C to 38°C is standard, but the temperature may be made higher so as to accelerate the processing or to shorten the processing time, or on the contrary, the temperature may be made lower so as to improve the quality of images formed and to improve the stability of the processing solutions used. For the purpose of economization of silver in the photographic materials, the cobalt intensification or hydrogen peroxide intensification described in DE-C-2,226,770 and US-A-3,674,499 may be employed in the processing the photographic materials of the invention. The silver halide emulsion of the present invention which has been explained in detail hereinabove contains tabular silver halide grains having a completely uniform silver iodide distribution. In accordance with the present invention, therefore, there is provided a negative silver halide emulsion having excellent photographic characteristics in terms of sensitivity, gradation, graininess, sharpness, resolving power, covering power, storability, latent image stability and pressure-resistance. The following examples are intended to illustrate the present invention in more detail but not to limit it in any way. EXAMPLE 1 Preparation of Emulsion (1-A) Containing Fine Silver iodobromide Emulsion: 1200 ml of a 1.2 M silver nitrate solution and 1200 ml of an aquenous halide solution containing 1.08 M potassium bromide and 0.12 M potassium iodide were added to 2.6 liters of a 2.0 wt.% gelatin solution containing 0.026 M potassium bromide, with stirring by the double jet method, over a period of 15 minutes, whereupon the gelatin solution was kept at 35°C. Afterwards, the resulting emulsion was washed by the conventional flocculation method, 30 g of gelatin were added thereto and dissolved, and then the emulsion was adjusted to have a pH of 6.5 and a pAg of 8.6. The thus obtained fine silver iodobromide grains (silver iodide content: 10%) had a mean grain size of 0.07 µm. Preparation of Tabular Silver Bromide Nuclear Grains (1-B): 150 ml of a 2.0 M silver nitrate solution and 150 ml of 2.0 M potassium bromide solution were added to 1.3 liters of a 0.8 wt.% gelatin solution containing 0.08 M potassium bromide, with stirring by the double jet method, whereupon the gelatin solution was kept at 30°C. After the addition, such was elevated up to 70°C and 30 g of gelatin were added thereto. Afterwards, this was ripened for 30 minutes. The thus formed tabular silver bromide grains which are to be nuclei (hereinafter referred to as "seed crystals") were washed by the conventional flocculation method, and then adjusted to have a pH of 6.0 and a pAg of 7.5 at 40°C. The mean projected area circle-corresponding diameter of the thus obtained tabular grains was 0.4 µm. Preparation of Tabular Silver Iodobromide Emulsion (1-C) (Comparative Emulsion): 1/10 of the above mentioned seed crystals were dissolved in one liter of a solution containing 3 wt.% of gelatin, and the resulting solution was adjusted to have a temperature of 75°C and a pBr value of 1.1. Afterwards, 1 g of 3,6-dithioctane-1,8-diol was added thereto, and immediately an aqueous solution containing 150 g of silver nitrate and a potassium bromide solution containing 94.5 g of potassium bromide and 14.6 g of potassium iodide were added thereto by the double jet method under the condition of an equimolecularly accelerated flow rate (the final flow rate was 10 times of the initial flow rate), over a period of 80 minutes. Afterwards, the resulting emulsion was cooled to 35°C and washed by the conventional flocculation method. Then such was adjusted to have a pH value of 6.5 and a pAg value of 8.6 at 40°C and stored in a cold dark place. The grains formed had a mean projected area circle-corresponding diameter of 2.2 µm and a mean thickness of 0.3 µm. Preparation of Tabular Silver Iodobromide Emulsion (1-D) (Emulsion of the Invention): 1/10 of the above-mentioned seed crystals were dissolved in one liter of a solution containing 3 wt.% of gelatin, and the resulting solution was adjusted to have a temperature of 75°C and a pBr value of 1.1. Afterwards, 1 g of 3,6-dithioctane-1,8-diol was added thereto, and immediately the fine grain emulsion (1-A) as dissolved was added thereto by a pump. The condition of the addition speed was the same as that in the preparation of the aforesaid Emulsion (1-C), and the dissolved emulsion (1-A) was injected into the reactor vessel via a pump over a period of 80 minutes. (Precisely, the total amount of the Emulsion (1-A) added was 150 g as silver nitrate, and the final flow rate of Emulsion (1-A) being added was 10 times of the initial flow rate thereof.) Next, the thus prepared emulsion was washed with water in the same manner as in the case of Emulsion (1-C), and then adjusted to have a pH value of 6.5 and a pAg value of 8.6 at 40°C. The tabular grains formed had a mean projected area circle-corresponding diameter of 2.2 µm and a mean grain thickness of 0.3 µm. Preparation of Tabular Silver Iodobromide Emulsion (1-E) (Emulsion of the Invention): Emulsion (1-E) was prepared in the same manner as Emulsions (1-C) and (1-D), except for the following. In order to prepare fine grains an aqueous solution containing 150 g of silver nitrate, a potassium bromide solution containing 10 mol% of potassium iodide (the potassium bromide therein being equimolecular to silver nitrate in the solution), and 250 ml of an aquoeus 3 wt.% gelatin solution were added to a strong and efficient mixer which was provided near the reactor vessel, by the triple-jet method over a period of 80 minutes, under the condition of an accelerated flow rate (the final flow rate was 10 times of the initial flow rate). The reaction mixture was stirred in the mixer to give ultra-fine grains, and these were directly continuously introduced into the reactor vessel from the mixer vessel. In the procedure, the temperature of the mixer vessel was kept at 35°C. Afterwards, the thus prepared emulsion was washed with water in the same manner as Emulsion (1-C) and then adjusted to have a pH value of 6.5 and a pAg value of 8.6 at 40°C. The tabular grains formed had a mean projected area circle-corresponding diameter of 2.2 µm and a mean grain thickness of 0.3 µm. Preparation of Tabular Silver Iodobromide Emulsion (1-F) (Comparitive Example) Emulsion (1-F) was prepared in the same manner as Emulsions (1-C), except that the pBr value was adjusted to be 2.6 during the growth of the grains and 3,6-dithioctane-1,8-diol was not added. In the thus prepared emulsion, 80 % of the tabular grains were hexagonal tabular grains having a mean projected area circle-corresponding diameter of 2.4 µm. The fluctuation coefficient of the grains was 19 %. The mean grain thickness of 0.22 µm. Preparation of Tabular Silver Iodobromide Emulsion (1-G) (Emulsion of the Invention): Emulsion (1-G) was prepared in the same manner as Emulsion (1-E), except that the pBr value was adjusted to be 2.6 during the growth of the grains and 3,6-dithioctane-1,8-diol was not added. In the thus prepared emulsion, 90 % of the tabular grains were hexagonal tabular grains having a mean projected area circle-corresponding diameter of 2.3 µm. That is, the emulsion was a monodispersed tabular silver iodobromide emulsion having a fluctuation coefficient of 15 %. The mean grain thickness of 0.22 µm. Preparation of Tabular Silver Iodobromide Emulsion (1-H) (Emulsion of the Invention): Emulsion (1-H) was prepared in the same manner as Emulsion (1-D), except that the pBr value was adjusted to be 2.6 during the growth of the grains and 3,6-dithioctane-1,8-diol was not added. During the procedure, all of the fine grains added did not dissolve, i.e., some of the fine grains still remained after addition of the fine grains-containing emulsion was added to the reactor vessel. Grains of each of Emulsions (1-c), (1-D), (1-E), (1-F) and (1-G) were sampled, cooled with a liquid nitrogen and observed with a 200 KV transmission electron microscope. As a result, in the transmitted images of the respective grains, the distinct regular ring-like stripe patterns as shown in Fig. 1 were observed in the samples of Emulsions (1-C) and (1-F), whereas such stripe patterns were not observed at all in the samples of Emulsions (1-D), (1-E) and (1-G) of the present invention. Accordingly, it is understood that the emulsions of the present invention contained tabular silver iodobromide grains having a completely uniform silver iodide distribution. Comparing Emulsion (1-D) and Emulsion (1-H), it is understood that a silver halide solvent is necessary so as to dissolve the/previously prepared/fine grains (1-A) having a mean grain size of 0.05 µm. (In this case, bromide ion and 3,6-dithioctane-1,8-diol were used as the silver halide solvent.) However, such silver halide solvent is not longer necessary when the seed crystals are prepared in a mixer vessel and the seed crystals prepared therein are directly introduced into the reactor vessel for growth of the intended tabular grains. This is apparent from the result of the grains of Emulsion (1-G). In addition, comparing Emulsion (1-F) and (1-G), it is understood that Emulsion (1-G) was a monodiserpsed tabular grain emulsion because of the elevation of the ratio of the content of hexagonal tabular grains and of the reduction of the fluctuation coefficient of the projected area circle-corresponding diameter distribution of the grains in the emulsion. Accordingly, it is further understood with that the process of the present invention is an ideal method for growing tabular grains because of the complete uniformity of the silver iodide distribution in the tabular grains to be formed. Sodium thiosulfate (6.9 x 10⁻⁶ mol/mol Ag), potassium chloroaurate (5.0 x 10⁻⁶ mol/mol Ag) and potassium thiocyanate (5.1 x 10⁻⁴ mol/mol Ag) were added to each of Emulsions (1-C) to (1-G) (pH 6.5, pAg 8.6) and optimally chemically sensitized at 60°C. After completion of the chemical sensitization, 4-hydroxy-6-methyl-1,3,3a,7-tetrazaindene (7 x 10⁻³ mol/mol Ag) was added thereto. Each of the resulting emulsions was coated on a polyethylene terephthalate support in an amount of 3 g/m² as silver. Next, the thus prepared samples were exposed to a blue light with a 2854K tungusten light source via a 419 nm interference filter for 1/10 second, developed with the following Developer (D-1) (20°C, 4 minutes), fixed with the following Fixer (F-1), rinsed in water and dried. Developer (D-1): 1-Phenyl-3-pyrazolidone 0.5 g Hydroquinone 20.0 g Ethylenediamine-tetraacetic Acid Disodium Salt 2.0 g Potassium Sulfite 60.0 g Boric Acid 4.0 g Potassium Carbonate 20.0 g Sodium Bromide 5.0 g Diethylene Glycol 30.0 g Water to make 1 liter (pH 10.0) Fixer (F-1): Ammonium thiosulfate 200.0 g Sodium Sulfite (Anhydride) 20.0 g Boric Acid 8.0 g Ethylenediamine-teraacetic Acid Disodium Salt 0.1 g Aluminium Sulfate 15.0 g Sulfuric Acid 2.0 g Glacial Acetic Acid 22.0 g Water to make 1 liter (pH 4.2) The sensitometry results are shown in Table 1 below. Emulsion Relative Sensitivity Fog Note 1-C 115 0.14 Comparative Example (using silver halide solvent) 1-D 240 0.16 Example of the Invention(Grains were grown by addition of a fine grain emulsion.) 1-E 290 0.15 Example of the Invention (Mixer was used. Silver halide solvent was used.) 1-F 100 0.15 Comparative Example (silver halide solvent was not used) 1-G 300 0.13 Example of the Invention (Mixer was used. Silver halide solvent was not used.) As is obvious from the results shown in Table 1 above, the emulsions of the present invention had an extremely higher sensitivity than the comparative emulsions. EXAMPLE 2 Preparation of Tabular Silver Bromide Core Emulsion (2-A): 30 ml of a 2.0 M silver nitrate solution and 30 ml of a 2.0 M potassium bromide solution were added to 2 liters of a 0.8 wt.% gelatin solution containing 0.09 M potassium bromide, with stirring by the double jet method, whereupon the gelatin solution in the reactor vessel was kept at 30°C. After the addition, the temperature of the reaction mixture in the reactor vessel was elevated up to 75°C, and 40 g of gelatin was added thereto. Afterwards, a 1.0 M silver nitrate solution was added to make the pBr 2.55. Next, 150 g of silver nitrate were added to the reactor vessel under the condition of an accelerated flow rate (final flow rate was 10 times of the initial flow rate), over a period of 60 minutes, while potassium bromide was simultaneously added/thereto/to make the pBr 2.55. Afterwards, the emulsion thus prepared was cooled to 35°C and washed with water by the conventional flocculation method. 60 g of gelatin were added thereto and dissolved at 40°C. Then, the emulsion was adjusted to have a pH value of 6.5 and a pAg value of 8.6. The tabular silver bromide grains thus formed had a mean projected area circle-corresponding diameter of 1.4 µm and a grain thickness of 0.2 µm. The emulsion was a monodispersed tabular grain emulsion having a fluctuation coefficient of 15 %. Preparation of Tabular Silver Iodobromide Emulsion (2-B) (Comparative Emulsion): Emulsion (2-A) containing silver bromide in an amount of 50 g as silver nitrate was added to 1.1 liters of water and dissolved. The resulting solution was adjusted to have a temperature of 75°C and a pBr value of 1.4 Afterwards, 1 g of 3,6-dithioctane-1,8-diol was added thereto, and immediately 100 g of silver nitrate and a potassium bromide solution containing 6.3 g of potassium bromide and 9.8 g of potassium iodide were added thereto under the condition of an equimolecular constant flow rate, over a period of 50 minutes. Next, the resulting emulsion was washed with water by the conventional flocculation method and was adjusted to have a pH value of 6.5 and a pAg value of 8.6. The tabular silver iodobromide grains thus obtained were composed of silver bromide in the center part and 10 M% silver iodide-containing silver iodobromide in the outer peripheral part. The mean projected area circle-corresponding grain diameter was 2.3 µm and the mean grain thickness was 0.26 µm. Preparation of Tabular Silver Iodobromide Emulsion (2-C) Emulsion (2-C) was prepared in the same manner as Emulsion (2-B), except for the following. In place of adding the aqueous silver nitrate solution and the aqueous halide solution to the reactor vessel, fine grain emulsion (1-A) was added thereto in an amount of 100 g as silver nitrate, at a constant flow rate over a period of 50 minutes. The tabular grains thus formed had a mean projected area circle-corresponding diameter of 2.5 µm and a mean grain thickness of 0.23 µm. Preparation of Tabular Silver Iodobromide Emulsion (2-D) Emulsion (2-D) was prepared in the same manner as Emulsion (2-B) and (2-C), except for the following. In order to prepare the core grain emulsion (2-A), 100 g of silver nitrate and a potassium bromide solution containing 6.3 g of potassium bromide and 9.8 g of potassium iodide were added to a strong and efficient mixer vessel which was provided near the reactor vessel, at an equimolecular constant flow rate. Prior to the addition of the solutions, 300 ml of a 2 wt.% gelatin solution were blended with the aqueous halide solution. The ultra-fine grains formed in the mixer were directly continuously introduced into the reactor vessel from the mixer vessel. In the procedure, the temperature in the mixer vessel was kept at 40°C. The tabular grains thus formed had a mean projected aea circle corresponding diameter of 2.4 µm and a mean grain thickness of 0.24 µm. Preparation of Tabular Silver Iodobromide Emulsion (2-E) Emulsion (2-E) was prepared in the same manner as Emulsion (2-D), except that the pBr value was adjusted to be 2.6 during the growth of th grains and 3,6-dithioctane-1,8-diol was not added. In the thus prepared emulsion, 86 % of the tabular grains were hexagonal tabular grains having a mean projected area circle-corresponding diameter of 2.1 µm. That is, the emulsion was a monodispersed tabular silver iodobromide emulsion having a fluctuation coefficient of 17 %. The mean grain thickness was 0.23 µm. Grains of each of Emulsions (2-B), (2-C), (2-D) and (2-E) were sampled, cooled with a liquid nitrogen and observed with a 200 KV transmission electron microscope. As a result, in the transmitted images of the respective grains, distinct regular ring-like stripe patterns were observed in the sample of Emulsion (2-B), whereas such stripe pattens were not observed at all in the samples of Emulsions (2-C), (2-D) and (2-E) of the present invention. Accordingly, it is understood that the emulsions of the present invention contain tabular silver iodobromide grains having a completely uniform silver iodide distribution. Fig. 3 shows photographs of Emulsions (2-B), (2-C) and (2-D) taken with a transmission electron microscope. In the grains shown in Fig. 3, the core is made of a pure silver bromide containing no silver iodide. Accordingly, any stripe patterns which would indicate the existence of non-uniform silver iodide are not observed in the core. The outer shell of the grains is made of a silver iodobromide phase containing 10 % of silver iodide. The core/shell ratio in the grains is 1/2. 250 mg/mol(Ag) of Sensitizing Dye (I) mentioned below were added to each of Emulsions (2-B) to (2-E) having a pH of 6.5 and a pAg of 8.6, at 60°C. 10 minutes after the addition, sodium thiosulfate (5.1 x 10⁻⁶ mol/mol Ag), potassium chloroaurate (4.5 x 10⁻⁶ mol/mol Ag) and potassium thiocyanate (5.1 x 10⁻⁴ mol/mol Ag) were added thereto for optimum chemical sensitization. After chemical sensitization, 100 g of each of Emulsions (2-B) to (2-E) (containing 0.08 mol of Ag) were melted at 40°C and the following compounds (1) to (3) were added thereto in order with stirring to give a coating composition. Sensitizing Dye (I): Next, the following substances (1) to (5) were blended in order with stirring at 40°C, to give a surface protective layer-coating composition. The thus prepared emulsion-coating composition and surface protective layer-coating composition were coated on a cellulose triacetate film support by the co-extrusion method, the volume ratio of the coated layers being 103/45. The amount of silver coated was 3.1 g/m². The samples thus prepared were wedgewise exposed with a light source (200 lux) having a color temperature of 2854K for 1/10 second and then developed with Developer (D-2) mentioned below at 20°C for 7 minutes. They were then fixed with Fixer (F-1), rinsed with water and dried. Developer (D-2): Metol 2 g Sodium Sulfite 100 g Hydroquinone 5 g Borax.5H₂O 1.53 g Water to make 1 liter The sensitometry results are shown in Table 2 below. Emulsion Relative Sensitivity Fog Note 2-B 100 0.16 Comparative Example 2-C 220 0.15 Example of the Invention 2-D 270 0.14 Example of the Invention 2-E 270 0.12 Example of the Invention As is obvious from the results in Table 2 above, the emulsions of the present invention had an extremely higher sensitivity than the comparative emulsion. EXAMPLE 3 Preparation of Fine Silver Iodobromide Grain Emulsion (3-A): 1200 ml of a 1.2 M silver nitrate solution and 1200 ml of an aqueous halide solution containing 0.9 M potassium bromide and 0.3 M potassium iodide were added to 2.6 liters of a 2.0 wt.% gelatin solution containing 0.026 M potassium bromide, with stirring by the double jet method, over a period of 15 minutes, whereupon the gelatin solution was kept at 35°C. Afterwards, the resulting emulsion was washed by the conventional flocculation method, 30 g of gelatin were added thereto and dissolved, and then the emulsion was adjusted to have a pH of 6.5 and a pAg of 8.6. The thus obtained fine silver iodobromide grains (silver iodide content: 25%) and a mean grain size of 0.07 µm. Preparation of Tabular Silver Iodobromide Emulsion (3-B) (Comparative Emulsion): 20 ml of a 2.0 M silver nitrate solution and 20 ml of an aqueous halide solution containing 0.5 M potassium iodide and 1.5 M potassium bromide were added to 1.3 liters of a 0.8 wt.% gelatin solution containing 0.02 mol of potassium bromide, with stirring by the double jet method, over a period of 30 seconds, whereupon the gelatin solution was kept at 30°. After the addition, the temperature of the reaction mixture was elevated up to 70°C and 30 g of gelatin was added thereto. Afterwards this was ripened for 30 minutes. Thus silver iodobromide nuclei grains containing 25 % of silver iodide were obtained. A silver nitrate solution was added thereto so that the pBr value was adjusted to be 2.0. Next, 75 g of a silver nitrate and potassium bromide solution containing 39.4 g of potassium bromide and 18.3 g of potassium iodide were added thereto, under the condition of an equimolecularly accelerated flow rate (the final flow rate was 10 times of the initial flow rate). The pBr value was adjusted to be 2.5, and then 75 g of silver nitrate and the same molar amount of potassium bromide were added to the emulsion under the condition of an equimolecularly accelerated flow rate (the final flow rate was 10 times of the initial flow rate), over a period of 20 minutes. Afterwards, the emulsion was cooled to 35°C and washed with water by the conventional flocculation method. 60 g of gelatin was added thereto and dissolved at 40°C, and the resulting emulsion was adjusted to have a pH of 6.5 and a pAg of 8.6. The thus prepared tabular grains had a core/shell structure (core/shell ratio of 1/1), in which the core was silver iodobromide having 25 mol% of silver iodide content and the shell was pure silver bromide. The tabular grains had a mean projected area circle-corresponding diameter of 2.0 µm and a mean grain thickness of 0.28 µm. Preparation of Tabular Silver Iodobromide Emulsion (3-C)(Emulsion of the Invention): Tabular silver iodobromide grain nuclei containing 25 mol% of silver iodide were formed as in the case of the preparation of Emulsion (3-B), and 40 ml of an aqueous 30 % potassium bromide solution and 0.8 g of 3,6-dithioctane-1,8-diol were added thereto. Immediately, fine grain emulsion (3-A) as dissolved was added thereto by a pump, in an amount of 75 g as silver nitrate under the condition of an accelerated flow rate (the final flow rate was 10 times of the initial flow rate). Afterwards, the pBr value of the resulting emulsion was adjusted to be 2.6, and 75 g of silver nitrate and the same molar amount of potassium bromide were added thereto under the condition of an accelerated flow rate (the final flow rate was 2 times of the initial flow rate). Next, the emulsion was washed with water and re-dispersed, as in the case of the preparation of Emulsion (3-B). The thus prepared tabular grains were also core/shell grains, in which the core was silver iodobromide containing 25 mol% of silver iodide and the shell was silver bromide. The projected area circle-corresponding diameter of the grains was 2.6 µm and the grain thickness thereof was 0.22 µm. Preparation of Tabular Silver Iodobromide Emulsion (3-D)(Emulsion of the Invention): Nucleation of tabular silver iodobromide nuclei containing 25 mol% of silver iodide was effected as in the case of the preparation of Emulsion (3-B). Next, 75 g of silver nitrate and a potassium bromide solution containing 39.4 g of potassium bromide and 18.3 g of potassium iodide were added to a strong and efficient mixer as provided near the reactor vessel, over a period of 40 minutes under the condition of an equimolecularly accelerated flow rate (the final flow rate was 10 times of the initial flow rate), and at the same time 300 ml of a 1 wt.% gelatin solution was added to the mixer. After the addition, 75 g of silver nitrate and the same molar amount of potassium bromide were further added thereto under the condition of an equimolecularly accelerated flow rate (the final flow rate was 2 time of the initial flow rate) over a period of 20 minutes, and at the same time 300 ml of a 1 wt.% gelatin solution was also added to the mixer. Next, the emulsion was washed with water and re-dispersed, as in the case of the preparation of Emulsion (3-B). The thus prepared tabular grains were also core/shell grains, in which the core was silver iodobromide containing 25 mol % of silver iodide and the shell was silver bromide. The mean projected area circle-corresponding diameter of the grains was 1.9 µm and the mean grain thickness thereof was 0.28 µm. Grains of each of Emulsions (3-B), (3-C) and (3-D) were sampled, and the transmitted images of the respective grains were observed in the same manner as mentioned above. In the grains of Emulsion (3-B), distinct regular ring-like stripe patterns were observed in the core silver iodobromode phase around the nucleus thereof. In Emulsions (3-C) and (3-D), such patterns were not observed. In all three emulsions, no stripe patterns were observed in the shell silver bromide phase. Each of emulsions (3-B), (3-C) and (3-D) was optimally sensitized with sodium thiosulfate (5.5 x 10⁻⁶ mol/mol Ag), chloroauric acid (4.0 x 10⁻⁶ mol/mol Ag) and potassium thiocyanate (3.0 x 10⁻⁴ mol/mol Ag) at 60°C, and 250 mg/mol (Ag) of the following Sensitizing Dye II was added thereto. Sensitizing Dye II: Next, 7 x 10⁻³ mol/mol Ag 4-hydroxy-6-methyl-1,3,3a-7-tetrazaindene were added to each of the thus sensitized emulsion. (Regarding Emulsion (2-C), the amount of the said compound to be added was properly adjusted so that the total amount of the compound being added could be same as that added to the other emulsions.) Each of the thus prepared coating compositions was coated on a polyethylene terepthalate support in an amount of 2 g/m² as silver. The thus coated samples were then exposed to a 5400K light source through a filter capable of cutting a light having a wavelength shorter than 500 nm for 1/10 second (minus blue exposure). Next, the thus exposed samples were developed with Developer(D-1) (mentioned in Example 1) at 20°C for 4 minutes, fixed with the aforesaid Fixer (F-1), rinsed in water and then dried. The sensitometry results are shown in Table 3 below. Emulsion Relative Sensitivity Fog Note 3-B 100 0.07 Comparative Example 3-C 125 0.07 Example of the Invention 3-D 140 0.07 Example of the Invention EXAMPLE 4 Emulsion (3-B), (3-C) or (3-D) obtained in Example 3 was optimally chemically sensitized with sodium thiocyanate (4.5 x 10⁻⁶ mol/mol Ag) , chloroauric acid (3.0 x 10⁻⁶ mol/mol Ag) and potassium thiocyanate (2.5 x 10⁻⁴ mol/mol Ag) at 60°C, and then the compounds mentioned below were added thereto. The thus prepared coating composition was coated on a subbing layer-coated triacetyl cellulose film support. (1) Emulsion Layer: (i) Emulsion (shown in Table 4 below) (ii) Coupler (iii) Tricresyl Phosphate (iv) Sensitizing Dye: Sodium 5-chloro-5'-phenyl-4-ethyl-3,3'-(3-sulfopropyl)oxacarbocyanine (v) Stabilizer: 4-Hydroxy-6-methyl-1,3,3a,7-tetrazaindene (vi) Coating Aid: Sodium Dodecylbenzenesulfonate (2) Protective Layer: (i) Sodium 2,4-Dichloro-6-hydroxy-s-triazine (ii) Gelatin The thus prepared samples were sensitometrically exposed and then processed by the following color development procedure. The density of the thus processed samples was measured through a green filter. The results of the photographic properties of the samples obtained are shown in Table 4 below. The development procedure comprised the following steps, which were conducted at 38°C. 1. Color Development 2 min 45 sec 2. Bleaching 6 min 30 sec 3. Rinsing in Water 3 min 15 sec 4. Fixation 6 min 30 sec 5. Rinsing in Water 3 min 15 sec 6. Stabilization 3 min 15 sec The processing solutions used in the respective steps had the following compositions. Color Developer: Nitrilotriacetic Acid Sodium Salt 1.0 g Sodium Sulfite 4.0 g Sodium Carbonate 30.0 g Potassium Bromide 1.4 g Hydroxylamine Sulfate 2.4 g 4-(N-ethyl-N-β-hydroxyethylamino)-2-methylaniline Sulfate 4.5 g Water to make 1 liter Bleaching Solution: Ammonium Bromide 160.0 g Aqueous Ammonia (28 wt.%) 25.0 ml Ethylenediamine-tetraacetic Acid Sodium Salt 130 g Glacial Acetic Acid 14 ml Water to make 1 liter Fixer: Tetrapolyphosphoric Acid Sodium Salt 2.0 g Sodium Sulfite 4.0 g Ammonium Thiosulfate (70%) 175.0 ml Sodium Brisulfite 4.6 g Water to make 1 liter Stabilizer: Formaldehyde 8.0 ml Water to make 1 liter The results of the photographic properties of the samples obtained are shown in Table 4 below. Emulsion Relative Sensitivity Fog Note 3-B 100 0.16 Comparative Example 3-C 135 0.15 Example of the Invention 3-D 145 0.16 Example of the Invention As is obvious from the results in Table 4, Emulsions (3-C) and (3-D) of the present invention had a higher sensitivity than comparative Emulsion (3-B). Next, these samples were subjected to pressure-resistance tests, in which the emulsion-coated film was bent (bending-resistance test) and the emulsion coated film was scratched with a thin needle (scratch-resistance test). As a result, extreme pressure desensitization was noted in Emulsion (3-B), while almost no pressure desensitization was noted in Emulsions (3-C) and (3-D). From the tests, it is noted that the pressure-resistance of the emulsion of the present invention was noticeably improved. EXAMPLE 5 Emulsion (1-E) obtained in Example 1 was optimally chemically sensitized by the conventional manner, using sodium thiosulfate (5.5 x 10⁻⁶ mol/mol Ag) , potassium chloroaurate (4.5 x 10⁻⁶ mol/mol Ag) and potassium thiocyanate (4.5 x 10⁻⁴ mol/mol Ag) . The resulting emulsion was used as the emulsion for the fourth layer of the Sample No. 104 in Example 1 of JP-A-62-215271 to prepare a photographic material sample. This was processed in the manner described in Example 1 of JP-A-62-215271. As a result, the sample was proved to have a good photographic property. EXAMPLE 6 Preparation of Emulsion (6-A) (Comparitive Emulsion) 80 ml of a methanol solution of 0.1 % 3,4-dimethyl-4-thiazoline-2-thione was added to a reactor vessel containing 1.2 liters of a 3.0 wt.% gelatin solution having 0.06 M potassium bromide, with stirring, and 50 ml of a 0.3 M silver nitrate solution and 50 ml of an aqueous halide solution of 0.063 M potassium iodide and 0.19 M potassium bromide were added thereto by the double jet method, over a period of 3 minutes. Thus, silver iodobromide grain nuclei having a mean projected area circle-corresponding diameter of 0.3 µm and a silver iodide content of 25 mol% were formed. Subsequently, 800 ml of 1.5 M silver nitrate and 800 ml of a halide solution containing 0.375 M potassium iodide and 1.13 M potassium bromide were added thereto also by the double jet method at 75°C over a period of 100 minutes. Next, the resulting emulsion was cooled to 35°C and washed with water by the conventional flocculation method. 70 g of gelatin was added, and the emulsion was adjusted to have a pH of 6.2 and a pAg of 8.8. The first coat layer (inner layer) was formed on the nuclei. In the emulsion formed, the grains were octahedral silver iodobromide grains having a mean projected area circle-corresponding diameter of 1.7 µm and a silver iodide content of 25 mol%. Next, the emulsion was used as a core emulsion, and a shell (second coat layer or outer layer) of silver bromide was formed over the core. The molar ratio of first coat layer/second coat layer was 1/1. The thus obtained emulsion grains were monodispersed core/shell octahedral grains having a mean projected area circle-corresponding diameter of 2.2 µm and a core silver iodide content of 25 mol%. Preparation of Emulsion (6-B) (Emulsion of the Invention): Necleation was effected in the same manner as in preparation of Emulsion (6-A) to obtain silver iodobromide grain nuclei having a grain size of 0.3 µm. Subsequently, fine grains-containing Emulsion (3-A) (silver iodide content: 25 mol%) was added thereto in an amount of 1.2 mols as silver, with a pump over a period of 100 minutes. The first coat layer (inner layer) was formed on the nuclei. Afterwards, the emulsion was cooled and washed with water, and this was adjusted to have the same pH and pAg values as those of Emulsion (6-A). Next, the emulsion grains were used as core grains, and 800 mℓ of silver nitrate solution (1.5 M of AgNO₃) and 800 mℓ of potassium bromide solution (1.5 M of KBr) were simultaneously added thereto over a period of 80 minutes in the reactor vessel by the double jet method so as to form a silver bromide shell (second coat layer or outer layer) over the core grains. The ratio of first coat layer/second coat layer was 1/1. The thus prepared grains were monodispersed core/shell octahedral grains having a mean projected area circle-corresponding diameter of 2.2 µm and a silver iodide core of 25 mol%. Preparation of Emulsion (6-C) Emulsion of the Invention): Necleation was effected in the same manner as in preparation of Emulsion (6-A), and 800 ml of 1.5 M silver nitrate, 800 ml of a halide solution containing 0.375 M potassium iodide and 1.13 M potassium bromide and 500 ml of an aqueous 2 wt.% gelatin solution were added to a strong and efficient mixer as provided near the reactor vessel, by the triple jet method over a period of 100 minutes, whereupon the temperature in the mixer was kept at 30C. The ultra-fine grains formed in the mixer were directly continuously introduced into the reactor vessel kept at 75°C. Thus, the first coat layer was formed on the nuclei in the reactor vessel. Next, a 1.5 M silver nitrate solution, a 1.5 M potassium iodide solution and a 2 wt.% gelatin solution were added to the mixer, and silver bromide shell (second coat layer) was formed over the first coat layer of the respective grains. The ratio of first coat layer/second coat layer was 1/1. The thus obtained grains were monodispersed octahedral core/shell grains having a mean projected area circle-corresponding diameter of 2.2 µm. Each of emulsions (6-A), (6-B) and (6-C) was optimally chemically sensitized with sodium thiosulfate (4.0 x 10⁻⁶ mol/mol Ag), potassium chloroaurate (3.0 x 10⁻⁶ mol/mol Ag) and potassium thiocyanate (3.0 x 10⁻⁴ mol/mol Ag), and the compounds mentioned below were added thereto. The resulting coating composition was coated on a subbing layer-coated triacetyl cellulose film support. (1) Emulsion Layer: (a) Emulsion: See Table 5 (b) Coupler: (c) Tricresyl Phosphate (d) Sensitizing Dye: Sodium 5-Chloro-5'-phenyl-4-ethyl-3,3'-(3-sulfopropyl)-oxacarbocyanine (e) Stabilizer: 4-Hydroxy-6-methyl-1,3,3a,7-t-tetrazaindene (f) Coating Aid: Sodium Dedecylbenzenesulfonate (2) Protective Layer: (a) Sodium 2,4-dichloro-6-hydroxy-s-triazine (b) gelatin These samples were sensitometrically exposed and then processed by the color development procedure mentioned below. The density of the thus processed samples was measured with a green filter. The results of the photographic properties of the samples are shown in Table 5 below. The development procedure comprised the following steps all of which were conducted at 38°. 1. Color Development 2 min 45 sec 2. Bleaching 6 min 30 sec 3. Rinsing in Water 3 min 15 sec 4. Fixation 6 min 30 sec 5. Rinsing in Water 3 min 15 sec 6. Stabilization 3 min 15 sec The processing solutions used in the respective steps were as follows. Color Developer: Nitrilotriacetic Acid Sodium Salt 1.0 g Sodium Sulfite 4.0 g Sodium Carbonate 30.0 g Potassium Bromide 1.4 g Hydroxylamine Sulfate 2.4 g 4(N-ethyl-N-β-hydroxy e thylamino)-2-methylaniline Sulfate 4.5 g Water to make 1 liter Bleacher: Ammonium Bromide 160.0 g Aqueous Ammonia (28%) 25.0 ml Ethylenediamine-tetraacetic Acid Sodium Salt 130 g Glacial Acetic Acid 14 ml Water to make 1 liter Fixer: Tetrapolyphosphoric Acid Sodium Salt 2.0 g Sodium Sulfite 4.0 g Ammoniumthiosulfate (70%) 175.0 ml Sodium Bisulfite 4.6 g Water to make 1 liter Stabilizer: Formaldehyde 8.0 ml Water to make 1 liter The results of the photographic properties of the respective samples obtained are shown in Table 5 below. Emulsion Relative Sensitivity Fog Note 6-A 100 0.15 Comparative Example 6-B 135 0.13 Example of the Invention 6-C 145 0.12 Example of the Invention From the results in Table 5, it is noted that Emulsions (6-B) and (6-C) of the present invention are extremely excellent because of the high sensitivity and low fog. Next, the samples were subjected to pressure test (bending test where the emulsion-coated films are bent). As a result, Emulsion (6-A) showed extreme pressure desensitization, while Emulsions (6-B) and (6-C) showed almost no pressure desensitization. From the test, therefore, it is noted that the pressure-resistance of the emulsions of the present invention was noticeably improved. EXAMPLE 7 Preparation of Octahedral Silver Iodobromide Emulsion (7-A) (Comparative Emulsion): 80 ml of 5 % 3,6-dithioctane-1,8-diol were added to 1.2 liters of a aqueous 3.0 wt.% gelatin solution containing 0.03 M potassium bromide with stirring, and an aqueous solution containing 100 g of silver nitrate and an aqueous solution containing 70 g of potassium bromide were simultaneously added thereto by the double jet method at 75°C. Thus, monodispersed octahedral silver bromide grains of 1.7 µm were obtained. Subsequently, the grains were used as cores, and 400 ml of a 1.5 M aqueous silver nitrate solution and 400 ml of an aqueous halide solution containing 0.15 M potassium iodide and 1.35 M potassium bromide were simultaneously added thereto by the double jet method, whereby a silver iodobromide shell with 10 mol% of silver iodide content was formed over the core. Next, the resulting emulsion was cooled to 35°C and washed with water by the conventional flocculation method. 85 g of gelatin was added and the emulsion was adjusted to have a pH of 6.2 and a pAg of 8.8. The thus prepared grains were monodispersed octahedral core/shell grains having a mean projected area circle-corresponding diameter of 2.2µm, in which the shell contained 10 mol% of silver iodide. Preparation of Emulsion (7-B) (Emulsion of the Invention): Cores having a mean projected area circle-corresponding diameter of 1.7 µm were prepared as in the case of Emulsion (7-A). 20 ml of 30 % potassium bromide was added thereto. The fine grains-containing Emulsion (1-A) of Example 1 (containing 10 mol% of silver iodide) was added in an amount of 0.6 mol as silver, with a pump, at a constant flow rate over a period of 50 minutes. Thus, core/shell grains were prepared a in the case of Emulsion (7-A). These were monodispersed octahedral core/shell grains having a mean projected area circle-corresponding diameter of 2.2 µm, in which the shell contained 10 mol% of silver iodide. Preparation of Emulsion (7-C) (Emulsion of the Invention): Silver bromide cores having a mean projected area circle-corresponding diameter of 1.7 µm were prepared as in the case of Emulsion (7-A). Next, 400 ml of an aqueous 1.5 M silver nitrate solution, 400 ml of an aqueous halide solution containing 0.15 M potassium iodide and 1.35 M potassium bromide and 200 ml of an aqueous 1 wt.% gelatin solution were simultaneously added to a strong and efficient mixer provided near the reactor vessel, by the triple jet method over a period of 50 minutes, whereupon the temperature in the mixer was kept at 35°C. The ultra-fine grains thus prepared in the mixer were directly continuously introduced into the reactor vessel kept at 75°C. The thus prepared grains were monodispersed octahedral core/shell (1/1) grains having a mean projected area circle-corresponding diameter of 2.2 µm, in which the core was silver bromide and the shell was silver iodobromide having 10 mol% of silver iodide. Each of the thus prepared Emulsions (7-A), (7-B) and (7-C) was optimally chemically sensitized with sodium thiosulfate (7.2 x 10⁻⁶ mol/mol Ag), potassium chloroaurate (5.6 x 10⁻⁶ mol/mol Ag) and potassium thiocyanate (5.5 x 10⁻⁴ mol/mol Ag). Using each of the resulting emulsions, photographic samples were prepared in the same manner as in Example 6. These samples were sensitometrically processed, and the photographic properties of the respective samples obtained are shown in Table 6 below. Emulsion Relative Sensitivity Fog Note 7-A 100 0.15 Comparative Example 7-B 290 0.14 Example of the Invention 7-C 350 0.15 Example of the Invention As is obvious from the Table 6, Emulsions (7-B) and (7-C) of the present invention ahd an extremely higher sensitivity than the comparative Emulsion (7-A). EXAMPLE 8 The following protective colloids were used in Example 8. P-1: Alkali-processed bone gelatin (mean molecular weight: 100,000) P-2: Low molecular weight gelatin (mean molecular weight: 10,000) P-3: Polyvinyl Alcohol (mean molecular weight: 70,000) P-4: Azaindene group-containing Vinyl Polymer x : y = 95 : 5 (mean molecular weight: 60,000) P-5: Copolymer of Acrylamide/1-Vinyl-2-methylimidazole x : y = 96.5 : 3.5 (mean molecular weight: 50,000) P-6: Copolymer of Acrylamide/1-Vinyl-2-methylimidazole/acrylic Acid x : y : z = 9 : 84 : 6 (mean molecular weight: 50,000) P-7: Thioether group-containing Vinyl Polymer x : y = 6 : 1 (mean molecular weight: 70,000) P-8: Hydroxyquinoline group-containing Vinyl Polymer x : y = 96 : 4 (mean molecular weight: 70,000) P-9: Polyvinyl Pyrrolidone (mean molecular weight: 50,000) P-10: x : y : z = 3 : 10 : 87 (mean molecular weight: 70,000) P-11: x : y : z = 4 : 10 : 86 (mean molecular weight: 60,000) P-12: Copolymer of Polyvinyl Alcohol/Polyvinyl Pyrrolidone x : y = 4 : 6 (mean molecular weight: 60,000) Preparation of Tabular Silver Iodobromide Emulsion (8-A) : 150 cc of a 2.0 M silver nitrate solution and 150 cc of an aqueous halide solution containing 1.8 M potassium bromide and 0.2 M potassium iodide were added to 1.3 liters of a 0.8 wt% gelatin (P-1) solution containing 0.08 M potassium iodide, with stirring by the double jet method, whereupon the gelatin solution was kept at 30°C. After the addition, such as elevated up to 70°C and 30 g of gelatin (P-1) was added thereto. Afterwards, this was ripened for 30 minutes. The thus formed tabular silver bromide grains which are to be nuclei (hereinafter referred to as "seed crystal") were washed by the conventional flocculation method, and then adjusted to have a pH of 6.0 and a pAg of 7.5 at 40°C. The mean projected area circle-corresponding diameter of the thus obtained batular grains was 0.4 µm Preparation of Tabular Silver Iodobromide Emulsion (8-B) (Comparative Emulsion): 1/10 of the above-mentioned seed crystals were dissolved in one liter of a solution containing 3 wt% of gelatin (P-1), and the resulting solution was adjusted to have a temperature of 75°C and a pBr value of 1.4. Afterwards, fine grains was fed from the mixer vessel as equipped near the reactor vessel as shown in Fig. 2 and the grains were grown. Specifically, an aqueous solution containing 150 g of silver nitrate, a potassium bromide solution containing 10 mol% of potassium iodide (the potassium bromide therein being equimolecular to silver nitrate in the solution), and 500 ml of an aqueous 3 wt% gelatin solution were added to the mixer which was provided near the reactor vessel, by the triple-jet method over a period of 55 minutes, under the condition of an accelerated flow rate (the final flow rate was 10 times of the initial flow rate). The residence time of the solutions added in the mixer vessel was 10 seconds. The rotation number of the stirring blades in the mixer vessel was 3000 room temperature. The thus formed fine silver iodobromide grains were observed with a direct transmission electron microscope at a magnification of 20,000 times. As a result, the mean grain size was 0.03 µm. The temperature of the mixer vessel was kept at 35°C, and the fine grains formed in the mixer were continuously introduced into the reactor vessel. Preparation of Tabular Silver Iodobromide Emulsion (8-C) (Emulsion of the Invention): Emulsion (8-C) was tried to be prepared in the same manner as emulsion (8-B), except that the temperature in the mixer vessel was changed to 15°C. However, when the temperature in the mixer vessel was adjusted to be 15°C, the gelatin solution gelled in the mixer vessel so that fine grains were not formed therein. In order to obtain fine grains having a small grain size, it is said necessary to lower the temperature in the mixer vessel. However, when the gelatin (P-1) was used as the protective colloid, it was found that formation of fine grains was impossible at such a low temperature. Preparation of Tabular Silver Iodobromide Emulsion (8-D) (Emulsion of the Invention) : The low molecular weight gelatin (P-2) was used as the protective colloid, in place of the gelatin (P-1). In this case using the gelatin (P-2), no gelation occurred even at 15°C and formation of fine grains was possible. Next, Emulsion (E) to (N) were prepared under the same condition (temperature of-mixer vessel: 15°C), using each of the protective colloid synthetic polymers (P-3) to (P-12). Physical data of the thus prepared emulsion grains were shown in Table 7 below. Form the results in Table 7, the following matters could be understood. (1) When the gelatin (P-1) is used as the protective colloid, fine grains are formed in the mixer vessel having a temperature of 35°C. The fine grains thus formed are, after introduced into the reactor vessel, rapidly dissolved to release silver ion and halide ions (bromide ion and iodide ion) because of the small grain size thereof, which then precipitate on the tabular grains (nuclear grains) as previously existing in the reactor vessel. Thus the tabular grains grow, whereupon the growing speed of the grains is determined by the dissolving speed of the fine grains. That is, the smaller the grain size of the fine grains, the higher the growing speed of the tabular grains. (this is because, the fine grains having a smaller grain size may more rapidly dissolve.) In order to obtain fine grains having a smaller grain size, it is most effective to lower the temperature in formation of the fine grains. As is understood from the results in Table 7, the grain size of the fine grains in Emulsion (8-B) is 0.03 µm so that the dissolution of the grains in the reactor vessel is slow and the fine grains still remained in the reactor vessel. Further, the grain size of the finally obtained tabular grains was also small. In order to obtain fine grains having a smaller grain size, it is considered most effective to lower the temperature in formation of the fine grains. However, when the gelatin (P-1) was used, this gelled at a low temperature of 15°C so that formation of fine grains was impossible at such a low temperature. (2) the problem can be overcome by using the low molecular weight gelatin (P-2) and the synthetic protective colloids (P-3) to (P-12) as the protective colloid. It is obvious from Table 7 that ultra-fine grains were prepared in the mixer vessel at a low temperature, using anyone of (P-2) to (P-12), and that the tabular silver iodobromide grains having a narrow grain size distribution were obtained, using the thus prepared ultra-fine grains.

Claims: A silver halide photographic emulsion comprising a dispersion medium and silver halide grains, characterized in that the silver halide grains include a silver halide phase containing 3 mol% or more of silver iodide, the microscopic non-uniformity caused by the silver iodide being such that the silver halide grains exhibit at most two lines at an interval of 0.2 µm in the direction perpendicular to the lines when observed by a transmission electron microscope, and the emulsion contains at least 60 wt.% of the said silver halide grains. The silver halide photographic emulsion of claim 1, in which tabular silver halide grains having a mean aspect ratio larger than 2 account for at least 50% of the total projected area of the silver halide grains. The silver halide photographic emulsion according to claim 1, in which the silver halide grains have no lines caused by microscopic non-uniformity of the silver iodide therein. The silver halide photographic emulsion according to claim 3, in which the silver halide grains have a core/shell structure of silver bromide and silver iodobromide. The silver halide photographic emulsion according to claim 4, in which the silver halide grains have a three-layered core/shell structure of silver bromide and silver iodobromide. The silver halide photographic emulsion according to claim 4, in which the proportion of silver iodobromide in each grain is from 5 to 95 mol%. The silver halide photographic emulsion according to claim 5, in which the silver iodide content in the silver iodobromide phase is from 3 to 45 mol%. The silver halide photographic emulsion according to claim 7, in which the silver iodide content in the silver iodobromide phase is from 5 to 35 mol%. The silver halide photographic emulsion according to claim 8, in which the silver iodide content in the total silver halide grains is 3 mol% or more. The silver halide photographic emulsion according to claim 7, in which the silver iodide content in the total silver halide grains is 5 mol% ore more. The silver halide photographic emulsion according to claim 2, in which the tabular silver halide grains have a mean aspect ratio of from 3 to 20. The silver halide photographic emulsion according to claim 1 or 2, which has been prepared by using a low molecular gelatin, a synthetic high molecular compound having a protective colloidal action on silver halide grains or a natural high molecular weight compound except for gelatin as a binder. The silver halide photographic emulsion as in claim 12, in which the binder is a low molecular weight gelatin. A process for preparing the silver halide photographic emulsion according to claim 1, comprising the steps of (1) mixing an aqueous solution of a water-soluble silver salt and an aqueous solution of at least one water-soluble halide in a mixer disposed outside a reaction vessel and (2) charging an aqueous solution of protective colloid into the mixer at a concentration of at least 1 wt.% in at least one of the following methods: (a) singly; (b) in the aqueous solution of the water-soluble silver salt; and (c) in the aqueous solution of the water-soluble halide, for preparing silver halide seed grains and (3) mixing the silver halide seed grains with fine iodide-containing silver halide grains in a reactor vessel rather than adding separate aqueous halide and silver salt solutions thereto. A process for preparing the silver halide photographic emulsion according to claim 14, in which the mean grain size of the fine grain silver halide emulsion is 0.1 µm or less, the temperature during the formation of the fine grain silver halide emulsion is 40° C or less, and the grains are grown in a reaction vessel at a temperature of 50° C or more.

Inventors: URABE SHIGEHARU, URABE, SHIGEHARU
Applicants: FUJI PHOTO FILM CO LTD, FUJI PHOTO FILM CO., LTD.
Application year: 1989
Main classification: G03C1
IPC codes: G03C1