Abstract:
Disclosed is an observation apparatus capable of eliminating in a suitable manner astigmatism and chromatic aberration generated in an observed image. The apex angle θ of a contact prism is input by operating an apex angle setting knob of a control panel, and the attachment angle thereof is input by operating an attachment angle setting knob. Based on the observation magnification recognized and the input apex angle θ, a control device determines the astigmatism correction amount for a left observation optical system and determines the astigmatism correction amount for the right observation optical system. Further, based on the input attachment angle, the axial angles of the astigmatisms of the left observation optical system and the right observation optical system are determined. Next, variable cross cylinder lens rotation drive devices are respectively controlled to rotate the respective cylinder lenses of the left and right observation optical systems to achieve the axial angle and correction amount as determined.

Description:
RELATED APPLICATIONS  
       [0001]     This application is a divisional application of U.S. patent application Ser. No. 10/862,451, filed Jun. 8, 2004, which application claims priority under 35 U.S.C. § 119 of Japanese Application No. 2003-163931, filed Jun. 9, 2003, which are incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to an ophthalmologic operation microscope, and more particularly to an ophthalmologic operation microscope equipped with a front lens for observing the eyeground of the eye to be examined.  
         [0004]     2. Description of the Related Art  
         [0005]     Usually, an ophthalmologic operation is conducted with microscopic observation. JP2002-350735A (paragraph [0016]-[0018],  FIG. 6 ,  FIG. 7 , and  FIG. 9 ) discloses an example of a microscope for such ophthalmologic operation. In the ophthalmologic operation microscope as disclosed in the above publication, a front lens is arranged between the objective lens and the eye to be examined, making it possible to illuminate the interior of the eye. This enables the operator to perform operation, in particular, eyeground operation, with surgical instruments in both hands. As shown in  FIGS. 6 and 9  of the above-mentioned publication, the front lens is detachably arranged between the objective lens and the eye.  
         [0006]     Further, as shown in  FIG. 7  of the above-mentioned publication, in the ophthalmologic operation microscope as disclosed therein, illumination light is applied to the eye through an illumination prism provided so as to be offset from the optical axis of the observation optical system.  
         [0007]     When, however, illumination light is applied from a direction oblique to the optical axis of the observation optical system, there are observed two reflection images of the exit pupil of the illumination optical system due to the reflective action of the refraction surfaces of the front lens. That is, the front lens has two refraction surfaces, and, when seen from the direction of the optical axis of the observation optical system, the reflecting positions of the two refraction surfaces reflecting the exit pupil differ from each other, with the result that two reflection images are observed.  
         [0008]     Such reflection images lead to deficiency in visual field for the operator and constitute light spots in the observation image, making it rather difficult to observe the portions around the same.  
       SUMMARY OF THE INVENTION  
       [0009]     The present invention has been made in view of the above problem in the prior art. It is an object of the present invention to provide an ophthalmologic operation microscope that enables the operator to obtain a satisfactory visual field.  
         [0010]     In order to achieve the above-mentioned object, according to a first aspect of the present invention, there is provided an ophthalmologic operation microscope including: an objective lens arranged so as to be opposed to an eye to be examined; an observation optical system composed of various optical elements and used to observe the eye through the objective lens; an illumination optical system applying illumination light to the eye from a direction at a predetermined angle with respect to an optical axis of the observation optical system; a front lens detachably provided between the eye and the objective lens; and an image position changing means for changing positions of two reflection images of an exit pupil of the illumination optical system, which are generated through reflection of the illumination light by two refraction surfaces of the front lens, so that the two reflection images may not obstruct observation.  
         [0011]     Further, in order to achieve the above-mentioned object, according to a second aspect of the present invention, there is provided an ophthalmologic operation microscope in the first aspect of the invention, characterized in that the image position changing means changes the positions of the reflection images by arranging the front lens in an inclined state.  
         [0012]     Further, in order to achieve the above-mentioned object, according to a third aspect of the present invention, there is provided an ophthalmologic operation microscope in the first aspect of the invention, characterized in that the image position changing means arranges the front lens so as to be at an acute angle with respect to an observation axis.  
         [0013]     Further, in order to achieve the above-mentioned object, according to a fourth aspect of the present invention, there is provided an ophthalmologic operation microscope in the first aspect of the invention, characterized in that the image position changing means arranges the front lens so as to be at an obtuse angle with respect to an observation axis.  
         [0014]     Further, in order to achieve the above-mentioned object, according to a fifth aspect of the present invention, there is provided an ophthalmologic operation microscope in the second aspect of the invention, characterized in that the observation optical system has left and right observation optical paths for respectively guiding observation light to the left and right eyes of an operator, and that the image position changing means arranges the optical axis of the front lens in a direction so as to substantially divide in two equal parts an angle made by a segment connecting a midpoint of centers of left and right entrance pupils corresponding to the left and right observation optical paths and a focus of the observation light due to the front lens and a segment connecting a center of the exit pupil of the illumination optical system and the focus.  
         [0015]     In order to achieve the above-mentioned object, according to a sixth aspect of the present invention, there is provided an ophthalmologic operation microscope including: an objective lens arranged so as to be opposed to an eye to be examined; an observation optical system composed of various optical elements and used to observe the eye through the objective lens; an illumination optical system applying illumination light to the eye from a direction at a predetermined angle with respect to an optical axis of the observation optical system; a front lens detachably provided between the eye and the objective lens; and a shielding means for shielding a part of the illumination light reaching an area where reflection images of an exit pupil of the illumination optical system generated through reflection of the illumination light by two refraction surfaces of the front lens are to be observed.  
         [0016]     Further, in order to achieve the above-mentioned object, according to a seventh aspect of the present invention, there is provided an ophthalmologic operation microscope in the sixth aspect of the invention, characterized in that the shielding means is arranged at a position optically conjugate with a point in an entrance pupil of the observation optical system with respect to the refraction surfaces of the front lens.  
         [0017]     Further, in order to achieve the above-mentioned object, according to an eighth aspect of the present invention, there is provided an ophthalmologic operation microscope in the sixth or the seventh aspect of the invention, characterized in that the shielding means switches between shielding and transmission of a part of the illumination light according to whether the front lens is being used or removed.  
         [0018]     Further, in order to achieve the above-mentioned object, according to a ninth aspect of the present invention, there is provided an ophthalmologic operation microscope in any one of the sixth to the eighth aspects of the invention, characterized in that the shielding means is a liquid crystal display arranged in the optical path of the illumination light and capable of displaying at a desired position a shielding area shielding the illumination light.  
         [0019]     In order to achieve the above-mentioned object, according to a tenth aspect of the present invention, there is provided an ophthalmologic operation microscope including: an objective lens arranged so as to be opposed to an eye to be examined; an observation optical system composed of various optical elements and used to observe the eye through the objective lens; an illumination optical system applying illumination light to the eye from a direction at a predetermined angle with respect to an optical axis of the observation optical system; a front lens detachably provided between the eye and the objective lens; an image position changing means for changing positions of two reflection images of an exit pupil of the illumination optical system, which are generated through reflection of the illumination light by two refraction surfaces of the front lens, such that the two reflection images are substantially superimposed one upon the other when observed; and a shielding means for shielding a part of the illumination light reaching an area of the reflection images caused to be substantially superimposed one upon the other when observed by the image position changing means. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]      FIG. 1  is a schematic external view of the general construction of an ophthalmologic operation microscope according to an embodiment of the present invention;  
         [0021]      FIGS. 2A through 2C  are schematic enlarged views of the general construction of an operator microscope in an ophthalmologic operation microscope according to an embodiment of the present invention, in which  FIG. 2A  is an external side view,  FIG. 2B  is an external front view, and  FIG. 2C  is a see-through side view showing how a front lens is accommodated;  
         [0022]      FIGS. 3A through 3C  are enlarged views for illustrating the way the front lens is arranged in an ophthalmologic operation microscope according to an embodiment of the present invention, of which  FIG. 3A  is a see-through side view,  FIG. 3B  is a rear view, and  FIG. 3C  is an explanatory view illustrating how the front lens is arranged in an inclined state;  
         [0023]      FIG. 4  is a schematic side view showing the construction of an optical system of an ophthalmologic operation microscope according to an embodiment of the present invention;  
         [0024]      FIG. 5  is a top view schematically showing the construction of a part of the optical system of an ophthalmologic operation microscope according to an embodiment of the present invention;  
         [0025]      FIG. 6  is a schematic diagram showing the structure of a shielding member of an ophthalmologic operation microscope according to an embodiment of the present invention;  
         [0026]      FIGS. 7A and 7B  are schematic diagrams showing how eye observation is performed by a conventional ophthalmologic operation microscope;  
         [0027]      FIGS. 8A and 8B  are schematic diagrams showing how eye observation is performed by an ophthalmologic operation microscope according to an embodiment of the present invention;  
         [0028]      FIGS. 9A and 9B  are schematic diagrams showing how eye observation is performed by an ophthalmologic operation microscope according to an embodiment of the present invention;  
         [0029]      FIGS. 10A and 10B  are enlarged views schematically showing the construction of a part of an ophthalmologic operation microscope according to another embodiment of the present invention, of which  FIG. 10A  is a side view and  FIG. 10B  is a see-through side view;  
         [0030]      FIG. 11A  is an explanatory view showing the construction of a main portion of an ophthalmologic operation microscope according to a third embodiment of the present invention;  
         [0031]      FIG. 11B  is a sectional view taken along the line A-A of  FIG. 11A ;  
         [0032]      FIGS. 12A and 12B  are schematic diagrams showing the effects of the third embodiment of the present invention; and  
         [0033]      FIGS. 13A and 13B  are diagrams schematically showing the construction of a shielding member in an ophthalmologic operation microscope according to fourth embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0034]     An ophthalmologic operation microscope according to an embodiment of the present invention will now be described in detail with reference to the drawings.  
       First Embodiment  
       [0035]     [General Construction of an Ophthalmologic Operation Microscope]  
         [0036]      FIG. 1  shows the general construction of an ophthalmologic operation microscope  1  according to the first embodiment. This ophthalmologic operation microscope  1  is equipped with a column  2  for supporting the apparatus, a first arm  3  one end of which is connected to the upper end of the column  2 , a second arm  4  one end of which is connected to the other end of the first arm  3 , a drive device  5  connected to the other end of the second arm  4 , an operator microscope  6  suspended from the drive device  5 , an assistant microscope  7  disposed adjacent to the operator microscope  6 , and a foot switch  8  for performing various operations with a foot. The operator microscope  6  and the assistant microscope  7  are driven three-dimensionally, i.e., vertically and horizontally, by the drive device S. Symbol E indicates an eye of the patient who is subjected to operation. Numeral  40  indicates a front lens disposed between the objective lens of the operator microscope  6  (described below) and the eye E.  
         [0037]     [Construction of the Operator Microscope] 
         [0038]      FIGS. 2A through 2C  are enlarged views for illustrating the construction of the operator microscope  6 .  FIG. 2A  is an external side view,  FIG. 2B  is an external front view, and  FIG. 2C  is a see-through side view showing how the front lens is accommodated. As shown in these drawings, the operator microscope  6  is equipped with a main body portion  6   a,  a lens barrel portion  10 , an inverter portion  20 , and a pair of eyepieces  30  ( 30 L and  30 R). In  FIG. 2B , the eyepieces  30  are omitted. The front lens  40  is connected to the lens barrel portion  10  through the intermediation of a retaining arm  41 , etc., and is detachably provided between the objective lens and the eye E (as described in detail below).  
         [0039]     Although not shown, the main body portion  6 a accommodates a control circuit for performing operation control on the operator microscope  6 , a drive device for effecting vertical fine adjustment of the barrel portion  10  by the control circuit, etc. The lens barrel portion  10  accommodates an optical system (described below), inclusive of the objective lens  11 , for illuminating and observing the eye E. The inverter portion  20  accommodates a well-known optical unit for converting an inverted image as observed into an erect image.  
         [0040]     [Construction of the Front Lens and the Periphery Thereof] 
         [0041]     Next, the construction of the front lens  40  and the periphery thereof will be described. As stated above, the front lens  40  is connected to the operator microscope  6  through the intermediation of the retaining arm  41 , etc. The front lens  40  is mounted to a retaining plate  41   a  formed at the distal end of the retaining arm  41 .  
         [0042]     The retaining arm  41  and the retaining plate  41   a  are rotatably connected together by an axle  41   b.  The retaining plate  41   a  is equipped with a beveled portion  41   c.  The retaining arm  41  is equipped with a front lens operating knob  42  for swinging the retaining arm  41 .  
         [0043]     The operator microscope  6  further includes an ascent/descent arm  71  with a fringe portion  71   a  at its top, a connection portion  71   b  connected to the lower portion of the ascent/descent arm  71 , an ascent regulating member  72  connected to the connection portion  71   b,  a connection knob  73  passed through the connection portion  71   b,  and an accommodating portion  74  detachably mounted to the ascent regulating member  72  and serving to accommodate the front lens  40  and the retaining arm  41 . The retaining arm  41  is pivoted to the accommodating portion  74  by an axle  74 a. Further, a coil spring  54  is mounted to the top portion of the retaining arm  41 . The reason for making the accommodating portion  74  detachable with respect to the ascent regulating member  72  is that it has to be detached therefrom after operation, etc. in order to sterilize the front lens  40  and the retaining arm  41 . Even with the front lens  40 , etc. removed there from, the microscope can be used as an ordinary operation microscope. In the following, the members mentioned in this paragraph may be collectively referred to as the front lens support portion.  
         [0044]     The main body portion  6   a  of the operator microscope  6  is equipped with a drive portion  75  for vertically driving an ascent/descent arm support member  76  supporting the ascent/descent arm  71 . The ascent/descent arm  71  is passed through the ascent/descent arm support member  76 . Further, due to the presence of the fringe portion  71   a,  the ascent/descent arm  71  is prevented from being detached and dropping from the ascent/descent arm support member  76 . Thus, as the ascent/descent arm  76  is vertically moved by the drive portion  75 , the front lens  40  is vertically moved, thereby changing its position relative to the objective lens  11 . Due to this arrangement, independently of the fine vertical adjustment of the lens barrel portion  10 , it is possible to vertically move the front lens  40  alone.  
         [0045]     Further, mounted to the lower portion of the main body portion  6   a  is an ascent regulating member  77  for regulating, together with the ascent regulating member  72 , the upward movement range of the front lens support portion. Formed in this ascent regulating member  77  is a connection hole  77   a  for connecting and securing the front lens support portion to the main body portion  6   a  by operating the connection knob  73 . To connect the front lens support portion to the main body portion  6   a,  the front lens support portion is raised to the uppermost position by the drive portion  75  (At this time, a protrusion  73   a  of the connection knob  73  and the connection hole  77   a  are mated with each other), and the connection knob  73  is rotated in a predetermined direction to fit the protrusion  73   a  into the connection hole  77   a.    
         [0046]      FIGS. 2A and 2B  show the front lens  40  of the operator microscope  6  in the state in which the lens has been inserted between the eye E and the objective lens  11  (i.e., when it is being used). When the use of the front lens  40  is stopped and the front lens is to be retracted, the operator grips a front lens operating knob  42  and upwardly swings the retaining arm  41  around the axle  74   a,  whereby the front lens  40  and the retaining arm  41  are accommodated in the accommodating portion  74 . Conversely, to bring the front lens  40  accommodated in the accommodating portion  74  into the state of use, the retaining arm  41  is swung downwards in a similar fashion.  
         [0047]     As shown in  FIG. 2A , when it is being used, the front lens  40  is arranged such that its optical axis is, directed at a predetermined angle with respect to the optical axis direction (vertical direction) of the objective lens  11 . This inclined arrangement of the front lens  40  is realized by the construction as shown in  FIGS. 3A through 3C . In  FIGS. 2A  thorough  3 C, the inclination of the front lens  40  is somewhat exaggerated for clarity of illustration.  
         [0048]      FIG. 3A  is a see-through side view schematically showing the construction of the front lens  40  and the retaining plate  41   a,    FIG. 3B  is a schematic view of the front lens  40  and the retaining plate  41   a  as seen from the left side in  FIG. 3A , and  FIG. 3C  is an explanatory view showing how the front lens  40  is arranged in an inclined state when in use.  
         [0049]     As shown in  FIG. 3B , the retaining plate  41   a  has a U-shaped portion, and the lower end portion of the retaining arm  41  is inserted into the region surrounded by the U-shaped portion, for example, as shown in  FIG. 11  (B), pivoted and elastically retained. Accordingly, the retaining plate  41   a  can be held at the desired inclination angle as shown in a virtual line  41   d  of  FIG. 3 (C). As will be described in detail below, these retaining arm  41  and retaining plate  41   a  constitute what is referred to as an image position changing means in the present invention.  
         [0050]     As shown in  FIG. 3C , when the retaining plate  41   a  thus formed is used, the rotating operation of the retaining plate  41   a  with respect to the retaining arm  41  arranged vertically is regulated by the elastic holding force, so that the front lens  40  is arranged in an inclined state.  
         [0051]      FIG. 2C  shows the front lens  40  as accommodated in an accommodating portion  74  (accommodated position). As shown in the drawing, the front lens  40  and the retaining arm  41  upwardly swung around the axle  74   a  are accommodated so as to extend in the longitudinal direction of the accommodating portion  74 . Further, the retaining plate  41   a  is rotated around the axle  41   b,  and accommodated in a folded state. This is due to the action of the beveled portion  41   c  of the retaining plate  41   a  and a contact member  74   b  mounted to an end of the accommodating portion  74 ; when the retaining arm  41  is swung upwards, the beveled portion  41   c  comes into contact with the contact member  74   b,  and the retaining plate  41   a  is rotated around the axle  41   b  while being guided by the beveled portion  41   c  to be automatically folded before being accommodated. Further, the accommodating portion  74  is equipped with a micro switch  65  for detecting whether the front lens  40  is accommodated or not; when the front lens  40  is accommodated in the accommodating portion  74 , a part of the retaining plate  41  a comes into contact with the micro switch to turn it on, and, when the accommodation is canceled, the contact state is also canceled, and the switch is turned off. The operation of this micro switch  65  will be illustrated with reference to a variation described below.  
         [0052]     [Construction of the Optical System] 
         [0053]     Next, the optical system accommodated in the barrel portion  10  of the operator microscope  6  will be described with reference to  FIGS. 4, 5  and  6 .  FIG. 4  is a side view schematically showing the construction of this optical system.  FIG. 5  is a top view schematically showing the construction of a part of this optical system.  FIG. 6  is a diagram schematically showing the construction of a shielding member  16  described below.  
         [0054]     As shown in  FIG. 4 , provided in the lens barrel portion  10  are an objective lens  11 , an observation optical system  12 , a light source  13 , a light guide  14 , a shielding plate  15 , a shielding member  16 , a lens unit  17 , a diaphragm  17   a,  and deflection mirrors  18   a,    18   b,  and  18   c.  The light source  13 , the light guide  14 , the shielding plate  15 , the shielding member  16 , the lens unit  17 , the diaphragm  17   a,  and the deflection mirrors  18   a,    18   b,  and  18   c  constitute what is referred to as an illumination optical system in the present invention. Further, although not shown, the lens barrel portion  10  also contains an imaging means, such as a CCD device, for photographing an observation image of the eye E. In the following, photographing by such an imaging means will also be considered as a form of observation.  
         [0055]     Further, between the eye E and the objective lens  11 , there is arranged the front lens  40  in use in a state in which the lens is inclined by a predetermined angle. Here, observation light from the eye E forms a focus F between the front lens  40  and the objective lens  11 . Thus, the eye E is observed as an inverted image; however, it is observed after being converted to an erect image by the optical unit in the inverter  20 , so that the requisite operability during operation is ensured.  
         [0056]     In the extension of the optical axis of the objective lens  11 , there are arranged the eyepieces  30 L and  30 R for the left and right eyes (see  FIG. 1 ) for the operator to observe the eye E. The observation optical system  12  is composed of a pair of observation optical systems  12 L and  12 R as shown in  FIG. 5 , and the observation optical systems  12 L and  12 R are composed of lens units including variable-power lenses for varying observation power, guiding observation light to the left and right eyepieces  30 L and  30 R. In  FIG. 4 , numeral  12   a  indicates an entrance pupil of the observation optical system  12 .  
         [0057]     The light guide  14  is formed with a bundle of optical fibers guiding illumination light from the light source  13 . The shielding plate  15  is an optical member arranged adjacent to an exit end  14   a  of the light guide  14  and adapted to shield a part of the illumination light from the exit end  14   a,  thereby making it possible to apply illumination light having a desired cross-sectional configuration. The shielding plate  15  is constructed so as to make it possible to selectively arrange light transmission areas of various configurations in the optical path for the illumination light by means of a shielding plate drive mechanism  15   a  consisting of a stepping motor or the like. The configuration of the light transmission area of the shielding plate  15  is selected according to the deflection mirror  18   a,    18   b,  or  18   c  that is used.  
         [0058]     The shielding member  16  constitutes what is referred to as the shielding means in the present invention, and consists, for example, of an optical member as shown in  FIG. 6  which is adapted to shield apart of the illumination light having passed the shielding plate  15 . This shielding member  16  is put in and out of the optical path of the illumination light by a shielding member drive means  16   a  consisting of a solenoid or the like. This operation can be effected, for example, by the foot switch  8 . The shielding member  16  shown in  FIG. 6  is a plate-like optical member formed of a plastic material or the like, and has a light transmitting area  16   b  transmitting illumination light and a shielding area  16   c  not transmitting illumination light. The position of this shielding area  16   c  will be discussed below.  
         [0059]     Further, the shielding member  16  is arranged such that, in an optical system in which a point A in the entrance pupil  12   a  of the observation optical system  12  (e.g., the center thereof) constitutes the object point and in which the front lens  40  constitutes the reflection surface, the shielding member is inserted at the position of a point B which is optically conjugate with respect to the object point.  
         [0060]     The lens unit  17  guides the illumination light transmitted through the light transmission area  16   b  of the shielding member  16  to a position in the vicinity of the optical axis of the observation optical system  12  (observation optical axis). The diaphragm  17   a  serves to restrict the area of the eye E to be illuminated, and is provided at a position conjugate with the focus F.  
         [0061]     The deflection mirrors  18   a,    18   b,  and  18   c  consist of reflection members arranged at predetermined positions above the objective lens  11  and in the vicinity of the observation optical axis, and serve to deflect the illumination light guided by the lens unit  17  and to guide it toward the eye E through the objective lens  11 .  
         [0062]     Next, the construction of the observation optical system  12  and its periphery will be described with reference to  FIG. 5 . As stated above, the observation optical system  12  consists of the left observation optical system  12 L and the right observation optical system  12 R; the left observation optical system  12 L guides observation light to the eyepiece  30 L for the left eye, and the right observation optical system  12 R guides observation light to the eyepiece  30 R for the right eye.  
         [0063]     The deflection mirrors  18   a,    18   b,  and  18   c  have configurations as shown in  FIG. 5 . The light transmission areas of the shielding plate  15  have configurations respectively corresponding to the configurations of the deflection mirrors  18   a,    18   b,  and  18   c.    
         [0064]     The deflection mirrors  18   a  and  18   b  are used when the red reflex (diaphanographic image) of the eye E is to be obtained. More specifically, the light transmission areas of the shielding plate  15  are appropriately selected to guide illumination light simultaneously to both the deflection mirrors  18   a  and  18   b,  whereby it is possible to illuminate the eye E from a small height with respect to the observation optical axis and from both sides. This makes it possible to obtain the red reflex over the entire observation area of the retina of the eye E.  
         [0065]     The illumination light applied by way of the deflection mirror  18   c  is at a larger angle with respect to that in the case of the deflection mirrors  18   a  and  18   b,  so that it is used when a sense of perspective is desired in the observation image.  
         [0066]     In the following, the case will be described in which illumination light is applied by way of the deflection mirror  18   c  as shown in  FIG. 4 . The following description also applies to the case in which the other reflection mirrors are used.  
         [0067]     In  FIG. 5 , symbols AL and AR respectively indicate the centers of the entrance pupils of the left observation optical system  12 L and the right observation optical system  12 R. The midpoint of the entrance pupils AL and AR will be indicated by symbol A mid . In the drawing, symbol C indicates the center of the entrance pupil of the illumination optical system when the deflection mirror  18   c  is being used.  
         [0068]     When seen sidewise, the arrangement of the midpoint A mid  of the entrance pupils AL and AR of the left and right observation optical systems  12 L and  12 R and a center C of the exit pupil of the illumination optical system is as shown in  FIG. 4 . That is, the illumination light from the light source  13  travels by way of the optical elements of the illumination optical system and is reflected by the area of the deflection mirror  18   c  including the center C of the exit pupil to illuminate the eye E through the objective lens  11  and the front lens  40 . The illumination light reflected by the eye E travels through the front lens  40  as observation light to temporarily form the focus F. Then, the light travels by way of the objective lens  11  and is transmitted through the entrance pupils AL and AR of the left and right observation optical systems  12 L and  12 R to be guided to the eyepieces  30 L and  30 R.  
         [0069]     Here, the segment passing the midpoint A mid  of the entrance pupils AL and AR and the focus F will be referred to as the observation axis O, and the segment in the angular direction when the illumination light reflected by the center C of the exit pupil passes the focus F will be referred to as an illumination axis L. Further, the angle made by an observation axis O and the illumination axis L will be referred to as α, and the segment dividing this angle a substantially into two equal parts will be referred to as a segment O′. That is, the angle α 1  is substantially equal to an angle α 2 . Here, an inclination angle θ of an optical axis O f  of the front lens  40  with respect to the observation axis O is set to α 1 .  
         [0070]     [Operation and Effects] 
         [0071]     In the following, the operation and effects of the ophthalmologic operation microscope  1 , constructed as described above, will be described with reference to  FIGS. 7A through 9B .  FIGS. 7A and 7B  schematically show how the eye E is observed in the conventional system, in which the optical axis O f  of the front lens  40  coincides with the observation axis O, with no inclination angle therebetween.  FIGS. 8A and 8B  schematically show how the eye E is observed according to the present invention, in which the front lens  40  is inclined with respect to the observation axis O.  FIGS. 9A and 9B  schematically show how observation is performed when the shielding member  16  is caused to operate in the case of  FIGS. 8A and 8B , in which the front lens  40  is inclined. Of these drawings,  FIGS. 7A, 8A , and  9 A show how observation is performed through the left observation optical system  12 L and  FIGS. 7B, 8B , and  9 B show how observation is performed through the right observation optical system  12 R.  
         [0072]     First, the conventional system shown in  FIGS. 7A and 7B  will be described. When the optical axis O f  of the front lens  40  coincides with the observation axis O, due to the angle a made by the illumination axis L and the observation axis O, the exit pupil of the illumination optical system forms two reflection images by the two refraction surfaces of the front lens  40 . Here, the two reflection images observed through the left observation optical system  12 L will be referred to as left reflection images P 1  and P 2 , and the two reflection images observed through the right observation optical system  12 R will be referred to as right reflection images Q 1  and Q 2 . When the angle α is of a magnitude normally selected, the left reflection images P 1  and P 2  and the right reflection images Q 1  and Q 2  are observed apart from each other. Thus, the left observation image of the eye E is encroached upon by the two light spots formed by the left reflection images P 1  and P 2 , with the result that the area that can be observed is reduced. Further, the portions around the two light spots suffer deterioration invisibility due to their dazzling. Similarly, the right observation image of the eye E suffers a reduction in the area allowing observation and deterioration in visibility in the portions around the light spots.  
         [0073]     In contrast, in the ophthalmologic operation microscope  1  of the present invention, the optical axis O f  of the front lens  40  is inclined with respect to the observation axis O by the above-mentioned angle θ, so that, as shown in  FIGS. 8A and 8B , the left reflection images P 1  and P 2  and the right reflection images Q 1  and Q 2  are respectively superimposed one upon the other when observed. Thus, the area of the observation image encroached upon by them is reduced (substantially by half), and the area that allows observation is enlarged as compared with that in the related art, thus achieving an improvement in terms of observing condition.  
         [0074]     Despite the improvement in observing condition through substantial coincidence of the two reflection images, the visibility of the portion around the reflection images is not yet to be considered as satisfactory. To eliminate this adverse influence, the shielding member driving means  1   6   a  is operated to put the shielding member  16  in the optical path of the illumination light.  
         [0075]     Here, the shielding region  16   c  of the shielding member  16  is positioned so as to shield the cross-sectional region of the illumination light reaching the position of the left reflection images P 1  and P 2  and the right reflection images Q 1  and Q 2 , which are substantially superimposed one upon the other when observed. Thus, the illumination light for forming each reflection image is partially shielded, so that, as shown in  FIGS. 9A and 9B , the positions corresponding to the left and right reflection images are observed as dark points  16   c ′ where no illumination light strikes. Thus, deterioration is eliminated in visibility due to the dazzling of the portions around the points  16   c ′, which are dark points (that is, the portions around the regions where the reflection images should be present).  
         [0076]     It is desirable that the shielding region  16   c  of the shielding member  16  be formed in such a size as will form a dark point slightly larger than the two reflection images substantially superimposed one upon the other when observed. The position of the shielding region  16   c  in the shielding member  16  is determined uniquely according to the inclination angle of the front lens  40 , so that it can be previously set at an appropriate position.  
         [0077]     While only the case has been illustrated in which the illumination light is reflected by the deflection mirror  18   c  to illuminate the eye E, what has been described is also applicable to the case in which observation is performed with the deflection mirrors  18   a  or  18   b  by adopting a construction allowing the inclination angle of the front lens  40  to be changed stepwise. Assuming that the angle made at this time by the illumination axis and the observation axis O is β, the front lens  40  is inclined such that its optical axis O f  is at an angle of approximately β/2 with respect to the observation axis O. Thus, it is only necessary for the optical axis O f  of the front lens  40  to be changed in position stepwise so as to be at α 1  or β/2 with respect to the observation axis O.  
         [0078]     Further, it is not always necessary for the inclining direction of the optical axis O f  of the front lens  40  to be substantially ½ of the angle made by the illumination axis L and the observation axis O; it can be appropriately set according to the conditions, such as the device specifications and the individual differences between devices.  
         [0079]     [Modifications] 
         [0080]     While in the above-described embodiment switching between use and non-use of the shielding member  16  is effected by operating the foot switch  8 , it is also possible to adopt a construction in which switching is effected in correspondence with use/non-use of the front lens  40 . For example, as shown in  FIG. 2C , the micro switch  65  is provided for detecting whether the front lens  40  is placed in the accommodation position or not (as described above). When the front lens  40  is released from the accommodation position and the micro switch  65  is turned off, a control circuit (not shown) provided in the main body portion  6   a  of the operator microscope  6  transmits a control signal to the shielding member driving means  16   a  to cause the shielding member  16  to be put in the optical path of the illumination light. Conversely, when the front lens  40  is set in the accommodation position and the micro switch  65  is turned on, the control circuit transmits a control signal to the shielding member driving means  16   a  to cause the shielding member  16  to retract from the optical path of the illumination light. Due to this construction, it is possible to set or remove the shielding member  16  in correspondence with use/non-use of the front lens  40 , which is convenient from the viewpoint of practical use.  
       Second Embodiment  
       [0081]     Next, another embodiment of the present invention will be described. This embodiment differs from the first embodiment in the construction for inclining the front lens. In the following description, the components that are the same as those of the first embodiment will be indicated by the same reference numerals.  FIGS. 10A and 10B  schematically show the construction of a part of the ophthalmologic operation microscope of this embodiment.  
         [0082]      FIG. 10A  shows the front lens  40  in the state of use. The ophthalmologic operation microscope of this embodiment is characterized by the construction of the accommodating portion  74  and the connection portion connected with the coil spring  54 . The accommodating portion  74  and the coil spring  54  are rotatably connected by an axle  74   a.  By swinging the retaining arm  41 , switching between use and non-use of the front lens  40  is effected. The portion where the accommodating portion  74  and the coil spring  54  are connected with each other is formed such that when the front lens  40  is being used, the retaining arm  41  is at a predetermined angle with respect to the vertical direction. To achieve such an inclination angle, there is adopted, for example, a construction as shown in  FIG. 10B , in which an inclined wall surface  74   c  is formed inside the accommodating portion  74 , with the rotation of the coil spring  54  in the direction of the position of use being restricted by this wall surface  74   c.  In this case, by adapting the inclination angle of the wall surface  74   c  with respect to the vertical direction to an appropriate inclination angle of the front lens  40 , it is possible to obtain the same effect as that in the first embodiment. It is to be noted here that, in  FIGS. 10A and 10B , the inclination angle of the front lens  40  is exaggerated to clarify the illustration.  
         [0083]     While two specific constructions for inclining the front lens  40  have been disclosed with reference to the first and second embodiments, it is possible to obtain the same effect as that of these embodiments by any other construction as long as it is a construction that causes the front lens  40  to incline with respect to the observation axis O.  
       Third Embodiment  
       [0084]     While in the first and second embodiments described above the front lens  40 , which is inclined, is arranged above the horizontal line, it is also possible to obtain the same effect as that of the first and second embodiments by inclining the front lens  40  so as to be situated below the horizontal line  1  as shown in  FIG. 1A .  
         [0085]     The third embodiment will be described in more detail. As indicated by the chain line in  FIG. 11A , in the first and second embodiments, the front lens  40  is inclined so as to be situated above the horizontal line  1 , which is at the level of the axle  41   b  (that is, such that the angle θ 1  made by the retaining arm  41  (or observation axis) and the retaining plate  41   a  is an acute angle). In this embodiment, in contrast, the front lens  40 ′, indicated by the solid line, is rotated in the direction of the arrow Y so as to be situated below the horizontal line  1  (that is, such that the angle θ 2  made by the retaining arm  41  (or observation axis) and the retaining plate  41   a ′ is an obtuse angle). To thus allow rotation between the acute angle θ 1  and the obtuse angle θ 2 , there is adopted, as shown in  FIG. 11B , a construction in which a click engagement through elastic contact is effected by a plurality of (at least two) recesses and protrusions  41   f  provided at the portion (in the vicinity of the axle  41   b ) where the retaining arm  41  and the front lens retaining plate  41   a ′ are connected and by a spring  41   e.  Alternatively, it is also possible to adopt a construction in which a wire is passed through the retaining arm  41 , with the lower end of the wire being fixed to the front lens retaining plate  41   a,  and in which the upper end of the wire is mounted to the apparatus main body through the intermediation of a pulley or the like, the operation being conducted manually. In the third embodiment, the above construction for effecting click engagement or wire connection constitutes the image position changing means.  
         [0086]     Next, the effect of the construction in which, as in the third embodiment, the front lens  40  is inclined so as to be situated below the horizontal line  1  will be illustrated with reference to  FIGS. 12A and 12B . Since the front lens  40  is inclined so as to be situated below the horizontal line  1 , the left reflection images P 1  and P 2  and the right reflection images Q 1  and Q 2  are respectively observed at positions spaced apart from each other as shown in  FIGS. 12A and 12B . As a result, the central area requisite for observation is enlarged, thereby achieving an improvement in terms of observation efficiency as in the first and second embodiments.  
       Fourth Embodiment  
       [0087]     Next, still another embodiment of the present invention will be described. This embodiment employs a shielding member different from that of the first embodiment.  FIGS. 13A and 13B  schematically show the construction of an example of the shielding member  160  of this embodiment. The shielding member  160  is a liquid crystal display (LCD) provided in the optical path of the illumination light; when nothing is being displayed, the illumination light is allowed to pass through it. Further, the shielding member  160  by LCD is controlled so as to be capable of displaying a black shielding area  160   c  at an arbitrary position by the above-mentioned control circuit. For example, it is possible to form a shielding area  160   c  consisting of a single area as shown in  FIG. 13A , or form a shielding area  160   c  consisting of two separate areas as shown in  FIG. 13B .  
         [0088]     Use of the shielding member  160  by LCD as the shielding member provides the following effects. First, when the front lens  40  is arranged in an inclined state and the two reflection images of the exit pupil of the illumination optical system as observed are substantially matched with each other, one shielding area  160   c  is displayed at the position on the shielding member  160  by LCD corresponding to the position of the reflection images, as shown in  FIG. 13A , to form a dark point in the observation image. When the front lens  40  is not inclined and two reflection images are to be observed, shielding areas are respectively displayed at the positions on the shielding member  160  by LCD corresponding to the positions of these two reflection images to thereby form two dark points.  
         [0089]     Thus, according to this embodiment, even when the position of the reflection image is changed as a result of a change in the incident height of the illumination light, etc., it is possible to appropriately change the position of the shielding area. Further, even when the front lens is not inclined, it is possible to turn the areas corresponding to the two reflection images into dark points, thereby achieving an improvement in terms of visibility.  
         [0090]     Further, it is also possible to adopt an arrangement in which the position on the shielding member  160  by LCD where the shielding area is displayed is changed in conformity with the position where the reflection image is observed by the above-mentioned control circuit (when the number of reflection images is one and/or two) With this arrangement, even if the position of the reflection image is changed, a dark point is automatically formed at that position, which is convenient from the viewpoint of practical use.  
         [0091]     The constructions described in detail above only constitute examples of how the present invention is to be carried out, and it goes without saying that various modifications and additions in terms of construction are possible without departing from the scope of the present invention.  
         [0092]     According to one aspect of the present invention, it is possible to change the positions of the two reflection images of the exit pupil of the illumination optical system due to the front lens such that they are substantially superimposed one upon the other when observed, whereby it is possible to enlarge the area that allows observation. Thus, the operator can obtain a satisfactory visual field.  
         [0093]     According to another aspect of the present invention, it is possible to shield a part of illumination light to be applied to the area where the reflection images of the exit pupil of the illumination optical system are observed, so that it is possible to eliminate a deterioration in visibility due to the dazzling of the reflection images, thereby providing a satisfactory visual field.