the epi - illumination optical system for microscopes of the invention is now explained with reference to some examples . in the following description , the epi - illumination optical system for microscopes will be called simply the epi - illumination optical system . fig1 is a taken - apart optical path diagram illustrative of the epi - illumination optical system for microscopes according to one example of the invention . the epi - illumination optical system here further comprises a microscope objective lens that serves also as a condenser lens , although that objective lens is left out . the epi - illumination optical system of the invention is a köhler illumination system using a collective lens array that is specifically built up of a light source 1 , a collimate lens 2 , a lens array 3 , an image position adjustment optical system 4 of negative power , a field stop 5 and a relay lens 6 of positive power . note here that the projection optical system is constructed of the image position adjustment optical system 4 and the relay lens 6 . divergent light from the light source 1 is collimated by the collimate lens 2 . the collimated light enters the lens array 3 having an exit - side lens surface in alignment with the back focal position . therefore , the collimated light incident on the lens array 3 is focused on the exit - side lens surface ; that is , an intermediate image ( the image of light source 1 ) is formed on the lens surface on the exit side . the intermediate image here is going to become an image ( a light source image array ) corresponding in sequence and number to the positive lenses ( microlenses ) involved . the image position adjustment optical system 4 forms a reduced virtual image of the intermediate image at a given position . light diverging from that reduced virtual image is focused on an exit pupil position 7 via the field stop 5 and relay lens 6 . note here that the exit pupil position 7 is the back focal position of the microscope objective lens ( not shown ). at that exit pupil position 7 here , the reduced virtual image is formed as a magnified real image this time . however , it is noted that the projection optical system ( image position adjustment optical system 4 and relay lens 6 ) is generally in the form of a reducing optical system . therefore , the intermediate image is reduced and projected onto the exit pupil position 7 of the microscope objective lens . and now , the exit pupil position 7 that is the back focal position of the microscope objective lens varies with the type ( e . g ., magnification ) of the microscope objective lens . for this reason , upon replacement of one microscope objective lens with another , there is the exit pupil position 7 changing too . as there is a variation in the exit pupil position ( back focal position ) 7 of the microscope objective lens , however , it causes the position of projection of the intermediate image to be going to be out of alignment with the exit pupil position ( back focal position ) 7 of another objective lens after replacement , producing illumination fluctuations that may otherwise get rid of the feature of the köhler illumination optical system . the same will be true , too , when an optical path splitting optical system for autofocusing or the like is inserted in the rear of the microscope objective lens . in the example here , therefore , a part of the projection optical system is designed to be movable in the optical axis direction to adjust the position of projection of the intermediate image . it is thus possible to align the position of projection of the intermediate image with the exit pupil position ( back focal position ) 7 of the microscope objective lens even when there is a different microscope objective lens used for it , or there is an optical path splitting optical system for auto - focusing or the like inserted in the rear of the microscope objective lens . in the example here , the projection optical system is in the form of a reducing optical system , as described above . for this reason , even when the collective lens array 3 is moved and adjusted in the optical axis direction to move the position of the intermediate image , the distance of movement of the image at the position of projection by the projection optical system is going to be shorter than the distance of movement of the collective lens array 3 . the reason is that an optical system has a longitudinal magnification equal to the square of a transverse magnification ; in a reducing optical system with a transverse magnification of less than 1 , its longitudinal magnification defined by the distance of its movement in the optical axis direction is supposed to become much less than its transverse magnification (& lt ; 1 ) rapidly . in the example here , therefore , a magnifying optical system rather the collective lens array 3 is designed to be movable . the “ magnifying optical system ” refers to an optical system which has positive refracting power that works converting divergent light into convergent light and in which the angle of divergence of that divergent light is greater than the angle of convergence of that convergent light . given the magnifying optical system , it is possible to increase the distance of movement of the image at the position of projection , even with a limited movement of that magnifying optical system . specifically , the relay lens 6 of the projection optical system defines the magnifying optical system ; the relay lens 6 is movable . the relay lens 6 , because of being a part of the projection optical system , may be called a partial optical system . on the other hand , the image position adjustment optical system 4 is in the form of a reducing optical system that is here restrained from movement . with the thus arranged example here , it is possible to align the position of the intermediate image with the exit pupil position 7 even upon a change in the exit pupil position ( back focal position ) 7 of the microscope objective lens . in the example of fig1 , the relay lens 6 is placed on a moving mechanism 20 . in the example here , the moving mechanism is a stage . the moving mechanism 20 is movable in a direction at least along the optical axis . reference numeral 21 is a controller adapted to control the moving mechanism 20 . in the example here , the relay lens 6 is moved by the moving mechanism 20 in the optical axis direction , as indicated by an action arrow a , to make its position adjustable . therefore , when there is replacement with another microscope objective lens or there is an optical path splitting optical system for autofocusing or the like inserted in the rear of the microscope objective lens , the position of the relay lens 6 in the optical axis direction may be adjusted to align the imaging position for the intermediate image with the exit pupil position ( back focal position ) 7 of the microscope objective lens . thus , as far as the movement of the relay lens 6 is concerned , its amount can be reduced . accordingly , the moving mechanism is unlikely to grow bulky . further , adjustment of the imaging position enables uniform köhler illumination of reduced fluctuations , even when there is replacement with another microscope objective lens or there is an optical path splitting optical system for autofocusing or the like inserted . fig2 is illustrative of a modification to the epi - illumination optical system for microscopes of fig1 . a difference with fig1 is that the lens array 3 , too , is movable and adjustable in the optical axis direction , as indicated by an action arrow b in fig2 . to move the lens array 3 , there are a moving mechanism 22 and a controller 23 provided . the controller 21 may serve also as the controller 23 . because the relay lens 6 is in the form of a magnifying optical system as described above , it is possible to change the imaging position for the intermediate image largely with a reduced or limited amount of movement . with the whole projection optical system in the form of a reducing optical system , on the other hand , there is no that large change in the imaging position for the intermediate image even when the lens array 3 is moved to some large degree . it is thus possible to implement adjustment of the imaging position for the intermediate image by movement and adjustment of the relay lens 6 , and implement a tweak of the imaging position for the intermediate image by movement of the lens array 3 . thus , if both are used , it is then possible to easily yet precisely align the imaging position for the intermediate image with the exit pupil position 7 even when the exit pupil position ( back focal position ) 7 of the microscope objective lens varies over a wide range . it is understood that the image position adjustment optical system in the projection optical system , too , is a reducing optical system . in other words , the aforesaid tweak may just as well be implemented by allowing the image position adjustment optical system 4 rather than the lens array 3 to be moved in the optical axis direction . fig3 is an optical path diagram including a part of the optical path in the microscope . in fig3 , reference numeral 11 is a band - pass filter , 12 a dichroic mirror , 10 a microscope objective lens , and 13 a sample plane . by the collimate lens 2 and lens array 3 , an intermediate image ( the image of light source 1 ) is formed on an end face at the exit side . the intermediate image is a light source image in an array form ( light source image array ). light from the intermediate image is converged at the relay lens 6 . the converged light focuses on the exit pupil position 7 ( fig1 and 2 ) via the band - pass filter 11 and dichroic mirror 12 . the exit pupil position 7 is the back focal position of the microscope objective lens 10 . the microscope objective lens 10 works also as a condenser lens . therefore , light focusing on the exit pupil position 7 provides an illumination to the sample plane 13 via the microscope objective lens ( condenser lens ) 10 . the sample plane 13 is located at the front focal position of the microscope objective lens 10 . therefore , the sample plane 13 is subjected to köhler illumination . the microscope objective lens 10 is an infinity correction system ; light from the sample plane ( sample ) 13 is collimated at the microscope objective lens 10 . the collimated light focuses at a given position by an imaging lens via an excited light cut filter , a reflecting mirror , etc . at the given position , an image of the sample plane ( sample ) is formed on a magnified scale . this image is magnified and viewed through an eyepiece lens or , alternatively , taken and measured through a taking optical system . in fig3 , the excited light cut filter , reflecting mirror and imaging lens are left out . for possible autofocusing onto a sample on the sample plane 13 , there is often a focus detection system provided . in that case , an optical path splitting optical system 15 is inserted in a slightly wide spacing between the microscope objective lens 10 and the dichroic mirror 12 , as shown in fig4 . for instance , movement of the objective lens side causes the exit pupil position 7 ( fig1 and 2 ) of the microscope objective lens 10 to move toward the side of the sample plane 13 . as a result , the exit pupil position 7 comes out of alignment with the position of projection of the intermediate image . accordingly , the relay lens 6 is moved in a direction indicated by an action arrow 8 in such a way as to approach the field stop 5 . thus , adjustment is implemented in such a way as to align the position of projection of the intermediate image with the thus moved exit pupil position 7 . accordingly , even when the optical path splitting optical system 15 is inserted , the exit pupil position 7 may be aligned with the position of projection of the intermediate image . it is thus possible to implement uniform illumination of reduced fluctuations . while the epi - illumination optical system for microscopes according to the invention has been described with reference to some embodiments or examples , it is understood that the invention is by no means limited to them and so many modifications may be possible . with the epi - illumination optical system for microscopes of the invention as described above , it is possible to cope with even when a plurality of objective lenses having varying exit pupil positions ( back focal positions ) are used , or there is an optical path splitting optical system or the like inserted , thereby making sure uniform köhler illumination of reduced fluctuations .