# Spectacles and Eyeglasses, their forms, mounting, and proper adjustment/Chapter 1

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By far the most generally useful method of placing glasses before the eyes is by spectacle frames, though the eyeglass, or pince-nez, has advantages in some cases, from the facility with which it may be placed in position or removed. The superiority of eyeglasses in appearance is another point not unworthy of consideration, as the glasses will surely be more constantly worn if they are becoming than if they are not so. Moreover, the patient is justly entitled to the correction of his refractive error with as little injury to his appearance as possible. The disadvantages of eyeglasses are, that for constant wear they are seldom so comfortable as spectacles; that on some faces it is nearly impossible to keep them in place; while, where the contained glass is cylindrical or prismatic, the rotary displacement which it is possible for the glass to take is a serious and sometimes fatal objection to their adoption.
Lorgnettes and single eyeglasses, or quizzing glasses, as they are called, are little more than playthings; though sometimes, as in aphakia, or high myopia, a strong convex or concave lens in one of these forms is of use when the spectacles constantly worn do not give the vision which may occasionally be required.
The Material of Spectacle Frames is usually gold, silver, steel, brass gold plated, or alloys containing nickel and tin. Horn, tortoise shell and celluloid find, or have found, a limited applicability in this connection.
Gold, of from 10 to 14 karat, is, by far, the best material for frames. Finer than this it is too flexible, while if less pure it may blacken the skin. In the end, such frames ​are cheaper than steel, as, owing to the liability of the latter metal to rust when in contact with the moist skin, the gold will outlast it many times over. In eyeglasses, however, the parts are heavier, and the metal is not in contact with the skin; so that there is not the same liability to rust. The gold frames furnished by opticians in this country usually have a stamp mark on the inner side of the right temple, near the hinge, which denotes the fineness of the gold: thus 8 karat is marked +; 10 karat, Θ; 12 karat,*; while 14 karat, or finer, is marked 14k, etc. “Gold filled” frames of the best quality and rather heavy stock are fairly rigid and durable and are considerably used, especially for children’s glasses.
Silver and brass are very poor material for frames, being soft, flexible and entirely lacking in elasticity. They are only useful for workmen’s protective goggles or some such purpose where very heavy frames are allow¬ able. The various alloys of nickel and tin, sold under trade names, have the faults of silver and brass in less degree. Some of them make fairly satisfactory frames for eyeglasses. For spectacles they are scarcely worth considering. Gold is, therefore, not only the best but the only good metal for frames. A cheap frame having a certain amount of rigidity and elasticity, and free from liability to rust, is still a thing to be desired.
Aluminum has been regarded as promising in this connection, but except lightness it has scarcely a quality to recommend it. It is soft, flexible and inelastic, and is readily corroded by the perspiration if the latter happen to be alkaline, which it frequently is. Moreover, aluminum cannot be soldered. In itself, therefore, it is unlikely ever to be available for this purpose. Some of its alloys, however, have interesting properties. That composed of ten parts of aluminum and ninety parts of copper some authorities assert to be the most rigid metal known. It is a red gold color, does not ​tarnish readily, is lighter than brass or gold, and can be soldered. It is possible that in some such alloy will be found a material having the valuable properties of gold for this purpose and without the latter’s cost. Largely in response to a demand for novelty celluloid or zylonite frames have been made in great variety. This substance is inflammable but can be bent in boiling water. As it is rather brittle, especialy when exposed to low temperatures, it is necessary to use it in thick pieces, but it is light in weight.
In Frames made wholly of this material it is nearly impossible to make any adjustment of the bridge, and rivets and screws do not hold very well. To meet these objections frames with metal hinges, bridges and temples, and finally metal frames covered with celluloid have appeared. The all celluloid frame with metal hinges appears to be the most popular one. With straight or half hook temples it is useful for glasses which are worn only in-doors, or for near work.
The Component Parts of Spectacles—A pair of spectacles is made up of fifteen or seventeen pieces, whose positions are shown in Fig. 1. They are: two lenses, two eye wires, four end pieces, two screws, two pins, or dowels, two temples, and one bridge. Sometimes the rings upon ​the temples, through which the dowels pass, are formed as separate pieces. Fig. 2 shows the name and position of each part of an eyeglass. A glance at the more important of the many interesting processes required in making these different parts will contribute to an understanding of the subject.
The Lenses.—The word lens is the Latin name of the lentil, a small bean. The resemblance in shape caused the name to be applied to the optical implement. Spectacle lenses are usually made of glass; sometimes of rock crystal (crystallized quartz). The latter substance has a slightly higher index of refraction, so that a lens of a given strength may be somewhat lighter when made of it than when made of glass. The notion is common that these “pebbles,” as they are called, possess a peculiar virtue in strengthening the eyes or in some other direction. I suppose the idea is that, being the product of Nature’s laboratory, they are necessarily superior. The advantage which they may have of being slightly lighter and harder than lenses of glass is more than counterbalanced by their higher cost, and by the fact that the index of refraction of rock crystal is not very constant.
Glass cannot be regarded as a definite chemical compound. It is of many varieties differing widely in composition. Ordinary glasses are mixtures of silicates, that is; ​silicic acid combined with some two or more of the metalic bases: sodium, calcium, potassium and lead. Ordinary optical glasses are divided into two types: (1) Crown glasses, which contain lime as one of their constituents and do not contain lead. (2) Flint glasses, which contain lead but no lime. A large number of special optical glasses have been produced by using other bases such as barium, magnesium, aluminum and zinc, either with or without the bases mentioned above. These glasses have optical properties valuable in various optical instruments but are not used in ophthalmic lenses.
The glass used for nearly all ophthalmic lenses is crown glass specially made for the purpose. Its essential qualities are: (1) Transparency and freedom from color. (2) Homogeneity. (Freedom from veins or striae of unequal refraction.) (3) Hardness and chemical stability. (4) Absence of internal strain. (Caused by unequal contraction of the outer and inner portions of a mass of glass while cooling.) (5) Constancy of its optical properties of refraction and dispersion. To attain these qualities in a high degree requires great care in the selection of raw materials of constant composition and their thorough admixture in constant proportions, as well as skill in the processes of melting, casting and rolling into sheets, annealing, cutting into pieces, and moulding into rough blanks for lenses.
Previous to the European war optical glass was not made in America. The curvature of the lensmaker’s tool is corelated with the density of the glass which he grinds. Hence a market once supplied with a satisfactory glass has a great incentive to adhere to its source of supply. This, and a little mystery with which the art of making optical glass was veiled, served to keep our opticians dependent on Europe in this regard. During the war the Government lent its aid in finding sources of pure raw materials and in experimental work of optical glass making. The War Industries Board in the Official Bulletin of June 21, ​1918, formally announced the successful production of optical glass in America and gave the Bausch & Lomb Optical Company full credit for the pioneer work and achievements. Apparently all of our lenses are now ground from American glass. Its index of refraction is 1.523.
Colored Glass.—Colored glasses are produced by adding metal oxides to glass while molten, each oxide producing a characteristic color. Such glass is graded by depth of color as A, B, C, and D; A being the lightest. The neutral tint called “smoke” is the result of using a number of the metalic oxides together. It is furnished in four shades and is the best color for use during mydriasis and in temporary photophobia from disease or after operations. Crookes glass is of a neutral tint lighter than the lightest shade of smoke. This color darkens somewhat, however, with age and exposure to light. The glass is an attempt to suppress the heat and ultra-violet rays without altering the color vision. It is suitable for temporary or occasional use when the eye is exposed to excessive light.
The two broad, polished surfaces of a lens are called its refracting surfaces, since it is at these surfaces that the rays of light are refracted when the lens is in use. On the shape of these surfaces, and their position relative to each other, depend all the powers and properties of a lens. Each of these surfaces may be either plane, spherical, or cylindrical. A spherical surface is such a one as, continued in all directions, would form a sphere, and which is, therefore, a segment of a sphere. Similarly, a cylindrical surface is the segment of a cylinder. Spherical and cylindrical surfaces may be either convex or cancave. A single surface of a lens may be, therefore, either
    Plane,
    Convex spherical,
    Concave spherical,
    Convex cylindrical,
    Concave cylindrical.
​Since every lens has two refracting surfaces, the list of lenses which it is possible for the lens maker to produce by combinations of these five primary surfaces is as follows:
* Prismatic.
* Plano-convex spherical.
* Plano-concave spherical.
* Plano-convex cylindrical.
* Plano-concave cylindrical.
* Biconvex spherical.
* Biconcave spherical.
* Concavo-convex (two varieties):
** Radius of curvature of convex surface greater than that of concave. (Converging meniscus.)
** Radius of curvature of convex surface less than that of concave. (Dispersing meniscus.)
* Sphero-cylindrical (four varieties):
** Convex sphere combined with convex cylinder.
** Convex sphere combined with concave cylinder.
** Concave sphere combined with concave cylinder.
** Concave sphere combined with convex cylinder.
* Biconvex cylindrical, axes coincident.
* Biconvex cylindrical, axes crossed.
* Biconcave cylindrical, axes coincident.
* Biconcave cylindrical, axes crossed.
* Concavo-convex cylindrical, axes coincident.
* Concavo-convex cylindrical, axes crossed.
Sections of lenses are shown in Fig. 3, each section illustrating two or more lenses, accordingly as we regard the curved lines as sections of spheres, or cylinders, and the straight lines as planes, or as sections of cylinders in the direction of their axes.
Lastly, the prism may be introduced as an element into each of these lenses. Thus we have quite a long list of the possible forms of the lens, and that without considering the “toric” surface, which will be spoken of later. Of these lenses only the nine first mentioned, and the combination ​of some of them with the prism, are in practical use. The others, the bicylindrical lenses, besides being difficult of manufacture, have each its optical equivalent in some simpler form of lens, either piano-cylindrical or sphero-cylindrical. They are only mentioned now because their use has been advocated by a few writers in the past.
Difficulties in grinding, and the near equivalence of certain of the lenses mentioned among the first nine of our list, render the use of some of these lenses quite rare. It is, for instance, more difficult to grind a perfect planospherical lens than it is to grind a bispherical, and as in weak lenses, such as are used in spectacles, the action of
every piano-spherical lens can be nearly exactly duplicated by some bispherical one, we seldom find piano-spherical lenses in use. Among sphero-cylindrical lenses also it is usual to consider certain combinations as equivalent to each other. For example, a convex spherical combined with a convex cylindrical, as equivalent to some stronger convex spherical combined with a concave cylinder. These lenses are only strictly equivalent, however, for a small area near their optical centers. When their influence on the field of vision is taken into account, they can no longer be considered identical, as we shall see in considering periscopic lenses.
In Fig. 3, the first three lenses shown act as convergers of rays, and are all considered as convex, or “plus” lenses, ​
being designated by the sign +, or sometimes by cx. The remaining lenses in the figure act as dispersers of the rays and are known as “minus,” or concave lenses, and receive the sign —, or sometimes cc. For the terms, spherical lens, cylindrical lens, prismatic lens, ​drical lens, etc., the words sphere, cylinder, prism, sphero-cylinder, etc., are frequently employed and are unobjectionable. Finally, the sign is used for “combined with” in the formula of a combination lens, as + 4.sphere + 2. cylinder.
The new system of numbering lenses, the dioptric system, has so entirely fulfilled the requirements of the users of lenses, and has so simplified and facilitated our every-day work and calculations, that the old or inch system of numbering is rapidly becoming of historical interest only. As its use, however, still survives in certain quarters, and lenses are frequently met with which are marked by this system, a table showing the equivalence of the ordinary lenses of the test case in the two systems is shown on page 17. It is calculated for an index of refraction of 1.53.
The simple apparatus used for grinding a single spherical lens is shown in Fig. 4. The disk of glass of which a lens is to be made is fastened, by means of pitch, to a small, cubical block of iron having a pit in the surface opposite that to which the glass is fastened. Into this pit fits a pin upon a lever, which is in the hand of the workman. When the free surface of the glass is applied to the surface of the “tool” to whose form it is to be ground, it, together with the block of iron, turns upon the pin. The universal joint at the end of the lever permits lateral and vertical movements, so that the workman is able to carry the glass freely over all portions of the tool.
The tool which gives the shape to the surface of the glass is made of steel; and for spherical glasses is in the form of a disk, with its surfaces looking upward and downward, and revolving about a vertical axis, like a potter’s wheel. The upper surface of this disk is convex for grinding concave glasses, or concave for grinding convex glasses. Of course, each strength of lens requires a separate tool having the requisite convexity or concavity of surface. The abrading material placed upon the surface of the tool ​is wet, powdered emery of successively finer and finer grades until the desired amount of glass has been ground away. When this process is complete, the surface of the glass has the desired spherical curvature, but it is rough: that is, it is “ground glass.”
To polish it; a piece of wet broadcloth or felt is smoothly applied to the surface of the tool upon which the glass was ground, conforming, of course, to that surface. The cloth, being sprinkled with wet “rouge” (a carefully calcined sulphate of iron), gives the glass held against it a beautiful polish without altering its spherical curvature. The same processes must now ​be gone over with the other surface of the lens, after which it is cleaned and cut to a shape suitable for its future mounting.
This is done by means of an implement called a lenscutter, in which the lens rests on a leather cushion and is held firmly in position by a rubber-tipped arm, while a diamond-tipped glass-cutter, guided by a pattern, traces the oval or other desired outline upon the glass. The superfluous glass is removed piecemeal by means of pincers, and the lens passes to the next process, which is the smoothing and, if necessary, beveling of the edges. This is done by hand upon large Scotch grindstones. If the lens is to be mounted in a round eye wire, its edge must be grooved by means of a file, while a skeleton frame will require the drilling of the glass, which may be done by hand with a steel drill or by a special machine.
In grinding a cylindrical lens the surface of the tool is, of course, a portion of the surface of a cylinder, and the glass is ground by a to-and-fro motion. It is evident that the position of the axis of the cylinder in the future spectacle need not be taken into account in grinding, but only in the process of cutting to shape for mounting.
When the lenses are of high power it is of advantage that they be made in the form of a meniscus, giving what are known as periscopic glasses. For instance, if a + 4. diopter lens is required, the anterior surface is ground to a + 6. D. and the posterior surfaces to a — 2. D. It is just as advantageous to a cylindrical or sphero-cylindrical glass to be periscopic as it is to a spherical, but under the previously described methods of grinding it is manifestly impossible to give them this form, as the cylinder is ground on one side, and the other ground to a plane or sphere, as the case may be.
To make a sphero-cylindrical lens in the form of a meniscus it is necessary to grind one surface to the toric form. The tore (Latin torus) is the surface ​dered by a circle which turns about an axis situated on the plane of the circle. A familiar example of the torus is the circular convex molding at the base of an architectural column. Fig. 5 shows the tool for rough-grinding a convex toric lens. The glass is ground on the inner, concave surface of the tool. Fig. 6 is the convex tool for grinding a concave tore. A glass ground upon a wheel having this form will present two cylindrical curves at right angles to each other, one depending on the radius of the wheel, and the other on the radius of the convexity of its rim. One surface of a lens being made toric the other is left for any desired spherical curve.
In practice it is the convex tores which are more often used, combined with concave spherical surfaces. A series of blanks is generally kept in stock with the convex toric side ready ground. These blanks usually have a base curve of + 6. (Sometimes of + 9.) That is, of the two curves of the toric surface the lower is + 6. The other curve is in series, beginning at + 6.12 and extending, perhaps, to + 14. The difference between the base curve and the other represents the cylindrical element in the future lens. If it is desired to produce, for example, a tone lens having the effect of + 2. Sph. + 1. Cyl., a blank is selected having its toric side ground +6. and +7., or, to state it differently, a + 1. Cyl. on a + 6. base curve. The other side of the blank is then ground - 4. sphere. If the effect of a simple convex cylinder is required, the concave side of ​the blank must be ground — 6. That is, equal to the base curve. If a minus effect is desired the curve of the concave side must be made greater than those of the toric side. For example, to produce an effect of — 2. Sph. O — 1. Cyl., using the blank before mentioned, it would be necessary to grind the concave side — 9. spherical. Blanks are furnished, however, having the inner surface ground to concave tores on a — 6. base curve.
Toric lenses, together with deep curved meniscus lenses, which are also popularly called torics, have almost displaced the old flat form of lenses in the better class of work.
Eye Wires, Temples, and Bridges.—Eye wires are made by wrapping the untempered wire, in the form of a spiral, closely about a metal cylinder. Being tempered while in this position, the loops of the spiral will retain the shape given them. A single cut down the side of the cylinder converts each loop into a separate ring End pieces and straight temples are stamped from sheets of metal, and afterward formed and tempered. Hook temples of steel are turned from wire upon a lathe. Bridges are usually made of oval or half-oval wire, and are simply pressed to the desired shape by a forming machine.
Of the Different Patterns of Spectacles.—In the common and strongest form of spectacle, the edge of the glass is beveled so as to enter a groove in the wire which surrounds it. In a second form, in which the edge of the glass is grooved for the reception of a fine, round wire, the object sought, of rendering the rim of the spectacles less conspicuous, is generally defeated by the fact that the glass must be made thicker than it otherwise need be, in order to give room for the groove on its edge. In concave glasses this is not the case, since the edge of the glass is here the thickest part, and such glasses may sometimes be mounted in this way with advantage. In a third form, called a “rimless” spectacles (Fig. 7), the wire encircling the glass is dispensed with altogether, small holes being drilled through ​the glass near its edge for the accommodation of screws which fasten the bridge and temples in place. The advantages of this form are its beauty and inconspicuousness.
It should never be prescribed for children, as it is quite liable to break at the point where the glass has been drilled. The edges of these glasses should not be polished, but should be given a dull finish, otherwise they reflect the light disagreeably.
Sides, or temples, have been variously constructed. Those having sliding and turn-pin joints are examples of ​antiquated forms. Those now used are the “hook,” or “riding-bow,” the “half-hook” and the plain, “straight” temple. The former are to be preferred in all cases where the glasses are to be worn constantly or nearly so, and the latter for those who wear glasses for near work only, and require to remove them frequently from the eyes. Hook temples are made in three lengths, 5½ inches, 6 inches, and 6½ inches, designated as short, medium, and long. These are sufficient for all cases. The half-hook is suitable for heavier temples and is used a good deal in celluloid frames.
By far the most useful spectacle bridge is the well known saddle bridge shown in Fig. 8. If properly made and of a sufficient length of wire it gives the fitter entire control of the position of the lenses in any given case and admits of nice adjustments in all its dimensions. The “K” bridge, formed of wires in the shape of the letter K, is allowable in some cases. The nearly similar “X” bridge allows the glasses to teeter, or see-saw across the nose, with the motions of the head. It is, however, the best form of bridge for reversible glasses; that is, glasses for persons having sight in one eye only, who may have their distant glass set in one side of a frame and their near glass in the other. By using this bridge and straight temples, or hook temples without a shoulder at the hinge, the spectacles may be turned over so as to bring either lens before the wearer’s seeing eye. The old-fashioned bridge, called the “curl,” is unobjectionable for cases in which the bridge of the nose is prominent, or for the spectacles of old people, who like to slip their glasses down toward the end of the nose. A small piece of cork is sometimes attached to the under side of the bridge where it comes in contact with the skin. It is unnecessary if the frames fit the face of the wearer properly. If it be desirable to remove all pressure from the bridge of the nose and transfer it to the sides, it may be done by soldering a pair of guards, similar to those used on eyeglasses, to the spectacle bridge. A much better ​way, however, is to use a special spectacle bridge of which there are several forms. One of the best of these is shown in Fig. 9. Its excellence consists in the shape of the wire to which the nose guards are soldered. It is such as to allow a wide range of adjustment.
The earliest spectacles appear to have had round eyes and late years have seen a revival of this shape in sizes ranging from 38 to 42 mm. diameter. It now disputes with the oval the first place in popular esteem. It is certainly the most convenient shape of any and is, perhaps, as generally becoming as any other.
The old standard oval eye has a difference of 9 mm. in its major and minor diameters. The vogue of large lenses, and of round lenses, has made the old ovals look antiquated and has led to the extensive use of shorter, broader ovals based on a difference of 5, 6, 7, or other number of mm. in the diameters. Each of these shapes, ​of course, has its own series of sizes. Table II shows the numbers and dimensions of the old standard ovals and also the diameter of the round lenses having equivalent areas. The larger sizes are known as 38, 40, 42 and 44 mm. ovals. Taking the 40 mm. oval for example, the name indicates that it will fit the same eye wire as the 40 mm. round lens.
It does not indicate the length or breadth of the oval, since these vary, depending on whether the oval is cut with 6, 7, 8, or 9 mm. of difference between its major and minor diameters. The “drop-oval” or “student” shape is one in which breadth is added to an oval below its horizontal diameter with the idea of better suiting the glass to overhanging brows or to persons who complain of seeing beneath their ​glasses. Sizes of these lenses may be named, like the true ovals, from the diameter of the round glass of equal circumference.
The octagon is an old shape which probably had its origin in the difficulty of cutting and mounting true ovals at an early stage of the optician’s art, and the demand for a narrow shape for presbyopes. The quest of novelty has led to an attempt to revive this shape.
Where glasses are used for near work only, the eyes are sometimes made of semi-oval shape, allowing the line of sight to pass over their upper, straight edge when the
wearer views a distant object. These are known as “half” “pulpit,” or “clerical” eyes. The increased use of bifocal lenses has almost driven them out of use.
Bifocal Glasses.—When glasses of different focusing power are required for distant and near vision, the trouble incident to frequent changing is obviated by “bifocal” glasses. That is, the lower part of the spectacle eye, which is used for near work, is made to differ in focusing power from the upper part, which is used for distant vision. Such bifocal glasses are also called Franklin glasses, from the philosopher who, as we have seen, invented them. ​The object sought may be attained in various ways. In the early Franklin glasses each eye contained two half-oval pieces, with their straight edges in apposition (A, Fig. 11). This has been improved upon by making the line of junction a curved one, giving somewhat greater latitude of distant vision and rendering the glass more secure in its frame. Both these forms are now obsolete. A more successful form of the two piece bifocal is the “cemented bifocal” shown at D, Fig. 11. To the back or front surface of the distance glass is cemented, by means of Canada balsam, a small lens whose strength, added to that of the
distance glass, equals the glass required for near work. The upper edge of the supplemental lens should be ground as thin as possible in order to render it inconspicuous. A special grade of these lenses is made by a patented process by which the supplemental lens is ground very thin by fastening it to a block of glass instead of one of iron, and grinding the two pieces of glass away together. These spectacles are strong, light and handsome, and may readily be made in the rimless form. For cylindrical lenses this arrangement is, moreover, cheaper than the others, since only the distance glass need have the cylinder ground upon it, the supplemental segment being a simple ​sphere. The changes in the correction for near which are likely to be needed from time to time are readily and cheaply effected in this form of bifocal glass.
The shape and size of the supplemental segment may be varied to suit all exigencies of use or taste. Fig. 12 illustrates some of the more common forms, of which B and C are the most useful.
The cemented bifocal lens still holds a place by reason of its cheapness and its ready adaptability to different cases and conditions. Its inherent weakness is the cement which is apt to become soft or brittle or opaque under the exigencies of use.
In still another form of bifocal glass, which did not go beyond the experimental stage, the small supplemental lens figured at D is made of flint glass of high refractive index and is countersunk, that is to say, is cemented into a corresponding concavity ground in the distance glass.
“Kryptok,” or fused bifocals, are a further evolution of the countersunk supplemental lens. In their manufacture a small lens of flint glass is let into a large lens of crown glass by countersinking, as described above. Instead of cementing the supplemental lens in position, however, the lenses are heated to the point of fusion of the glass, when its two portions unite. The surfaces of the glass are then ground. One surface of the small supplemental lens is exposed to the grinding and is reduced to the same curvature as the corresponding surface of the main lens. The necessary difference in the refraction of the upper and lower portions is dependent on the difference in index of refraction of the crown glass of which the main lens is composed and the flint glass of the supplemental lens.
This lens is made in both flat and meniscus form. The small supplemental lens is placed on the side away from the eye. In the meniscus formed lenses the optician keeps in stock “blanks” having the bifocal (convex) side ground and finished to some convenient spherical curve, ​+ 4., + 6., + 8., or + 10. A concave sphere or concave tore is then ground on the inner face of the glass.
“Ultex” bifocal glasses are made in one piece by a special method of grinding which produces two different spherical surfaces on one side of the lens. This is an old idea, and the old form of the glass is shown at C, Fig. 11. This old glass is very faulty from a prismatic effect inherent in the method of manufacture, and never came into general use.
The ultex glass is made in meniscus form only. The bifocal surface is the posterior or concave one. “Blanks”
have a base curve of — 4., — 6., — 8., or — 9. The lower part of the posterior surface, however, is ground on a separate tool to some less degree of concavity. The difference in curvature of the two portions of this concave surface represents the reading addition in the finished lens. The blanks are furnished in series with reading additions from .50 D. to 4.50 D.
To produce any given ultex bifocal the optician selects a blank having a suitable base curve with the proper reading addition and grinds on the front surface the required convex spherical curve. Where a cylindrical element is desired the front surface is given a convex toric form. For example, to produce an ultex glass + 1. sphere with + 2. sphere added for reading, a blank is selected whose base curve is — 6. and which has a + 2. ​reading addition. (That is, its reading portion has a 4. curve.) The front of this blank is then ground + 7.
To make a glass + 1. sphere + 1. Cyl., with + 2. sphere added for reading, the same blank would be used as in the last instance but the front surface would be made convex toric with its curves + 7- and + 8.
Both theoretically and practically the ultex lens represents the highest development of the bifocal idea. The fused glass is not so perfect optically or mechanically. When strong reading additions are made in this form the main lens is deeply countersunk and must be made correspondingly thick and heavy. In the process of fusing the two lenses together there is unavoidably some loss of definition at the joined surfaces. A more noticeable fault is the color aberration seen in some combinations, consequent on joining a convex flint lens of high color dispersion value to a concave crown lens of low dispersion. The ultex is free from these sources of trouble. It may be made as light as desired and objects seen through its reading portion are free from color aberration and with unimpaired definition. Since the one-piece bifocal has demonstrated its superiority to all other forms, lenses have been produced which avoid the use of the special machinery by which the ultex lenses are ground and polished. In these, the surface of the reading portion is sunk below the level of the surface of the main lens. There is thus a shoulder or inset at the junction. These lenses compete only with the cement bifocal, than which they are more durable but also more expensive.
Some persons declare that they cannot become accustomed to bifocals however well adjusted. Parallel, horizontal lines, as those of a staircase, are particularly confusing, it being possible to see each line doubled if the junction of the two segments of the glass is placed just opposite the pupil. Such persons may prefer having ​an “extra front” (Fig. 14): that is, a second pair of spectacles whose temples are replaced by short hooks, by means of which they are hung in front of the frame already upon the face. This is a rather clumsy device; less so, however, when the eyes of the extra front are made half oval instead of oval.
Eyeglasses.—The increased popularity of eyeglasses has stimulated invention and they now present a greater variety of forms than do spectacles. Every part is subject to such change and adjustment that it is quite possible to fit them to cases which were out of the question with the old forms.
An eyeglass is held in place by the pressure of a spring, or springs, upon the sides of the nose. There are four methods of placing this spring and hence four radically different forms of eyeglasses. First, we have the old and familiar form of arched spring (Figs. 15 and 16) which allows the frame to “open” while the lenses remain in their original plane. Second, the spring may be placed in a plane approximately at right angles to that of the lenses and the frame opens by the outer ends of the lenses moving forward. (Fig. 17.) Third, the lenses are joined by two bars sliding over each other in connection with a spiral spring. The frame opens when the two lenses are drawn apart along the line of their long axes. (Fig. 18.) Fourth, the lenses are joined by a rigid bridge, like a spectacle, while the nose-pieces are directly connected with spiral springs which press them to the sides of the nose. To open the frame special levers for the thumb and finger ​are provided by means of which the nose-pieces are pressed apart. (Fig. 19.) In another variety these levers are operated by pressing forward the outer ends of the lenses.
The first of these forms still holds its place as the most generally useful. It is simple and inconspicuous and the spring acts directly. For the majority of cases it is to be ​preferred. Its spring may be made light or heavy and may be offset from the plane of the lenses to accommodate overhanging brows. The second form offers no marked superiority over the first, from which it differs only in the direction of the action of the spring. This may make its hold more certain in a few cases, and occasionally persons who have tried both will prefer this form. The third and fourth forms have the same object in view, namely, to hold the lenses in position with the certainty of a spectacle frame, allowing none of that displacement of the axis of a cylinder, or base of a prism which is possible when they are joined by a yielding spring. The form designated in this description as “third,” and which is usually called the
“bar-spring” eyeglass and of which there are several variations (Fig. 18) has not met with much favor. They are heavy and cumbersome looking and there is apt to be lost motion between their sliding bars, which allows the displacement they are intended to prevent. The last of our four forms has more merit. Here, the junction between the lenses is really rigid, and no alteration of their relation to each other can take place. It is possible, how¬ ever, for one lens to be displaced upward and the other downward, or for one to stand forward and the other back¬ ward. This frame contains two springs, instead of one as in the other forms, and weakening of one spring may cause one of these displacements. Moreover, the small spiral springs are not very durable. In spite of its limitations, ​however, this frame has distinct value in some cases, especially where the lenses are heavy.
Nose-pieces are furnished in many forms. The use of the old form (Fig. 2) which lies in the same plane as the lenses, is now quite limited. The “offset guard” which bears upon the nose posterior to the plane of the lenses is much more generally useful. The latter should certainly be preferred for any case in which the glasses are to be worn for more than a few minutes at a time. There are dozens of varieties of the offset guard, many of which exist only for trade purposes and are advertised much beyond any peculiar merit which they possess.
    "A." For shallow bridge, prominent eyes, flat forehead.
    "B." For shallow bridge, prominent eyes and forehead.
    "C." The guard used for the average case.
    "D." Deep-set eyes, prominent nose and forehead.
    "E." Same as "C," but for lowering glasses (for reading).
    "F." Same as "B," but for lowering glasses (for reading).
    "G." Same as "C," but somewhat smaller and neater, although having less bearing surface.
Usually, the fewer and simpler the parts of any implement the better. The best guard is one stamped from a single piece of metal. Rivets will frequently loosen or fall out, or the metal may split to a rivet hole. As for pivots in nose-pieces, they are a delusion. What we should seek is not a self-adjusting eyeglass, but one capable of wide adjustment, and which will keep the shape which we give it. To this end the metal should be tough and pliable, though possessed of a certain amount of rigidity. The bearing surfaces of nose-pieces may be covered with cork, shell or celluloid. They soon become greasy and slippery, and beside being thick and clumsy, are not durable. If one tries to mold ​such nose-pieces to a different shape, the cork or shell frequently breaks, or the rivets pull out. The later patterns are stamped from a single sheet of metal, without any covering for the bearing surfaces, which are corrugated, fenestrated, or divided into two or more portions to give a more clinging hold upon the skin. In adjusting these nose-pieces to the patient’s face they may be shaped with the pliers with much greater freedom than can the other forms.
The “arm’’ or “foot” of the nose-piece is, in most forms, made in several lengths and shapes, for use on variously proportioned faces (Fig. 20). Much of the adjusting of eyeglasses is done by bending and twisting this arm.
Studs of eyeglasses are made in about six different lengths, from 1 mm. to 6 mm. as shown in Fig. 21. By means of these the intercentral distance of the lenses may be varied. In each of these lengths an offset stud is made, which may be used to place the lenses farther forward. Moreover, there are angular forms useful in tilting the lenses.
Spectacles for Cosmetic Effect.—Something may legitimately be done, at times, in the way of improving the appearance of a patient by the application of glasses. The ​blind whose eyes are not only sightless, but unsightly, very commonly hide them behind colored glasses. Neatly fit ting spectacles with large eyes of ground glass render the appearance of such persons less lugubrious. When one eye is useless for vision, and at the same time small, and the orbit undeveloped, a gratifying improvement in the appearance of the patient may be attained by placing before the shrunken eye a convex glass of sufficient strength to magnify it to the size of its fellow. The condition known as epicanthus can generally be removed by wearing eyeglasses whose nose-pieces draw just enough on the inner canthi to smooth out the offending fold of skin. As the subjects of epicanthus are generally flat-nosed, it may be necessary to furnish the eyeglasses with a pair of hook temples to keep them in place. Since operations for this disfigurement are so unsatisfactory, such an appliance is probably the best treatment we can advise in case the trouble is not outgrown.