main aperture 31 which is of an elongated configuration extending in the longitudinal direction and thin slit apertures 32a which are perpendicular to the longitudinal direction of the main aperture 31 the slit apertures 32a constitute alignment apertures the aperture plate 9 is disposed at an intermediate image forming point 30 of the endothelial cells of the cornea by the objective lens 8 of the photographing optical system 3 in operation the line of sight of the patient 39 s eye is at first fixed by making the eye to gaze fixedly at the fixed visual target 1 the illumination lamp 4 of the illuminating optical system is then turned on so as to illuminate the eye by a slit light beam the instrument is then adjusted in position so that the strong light ir reflected at the surface of the cornea is incident on the plate 9 the instrument is then finely adjusted so that the strong light ir reflected at the surface of the cornea leaks slightly through the slit apertures 32a in this manner only the reflections at the endothelial cells of the cornea can pass through the main aperture 31 in this position substantial part of the light reflected at the surface of the cornea is intercepted by the plate 9 under this condition the instrument is moved finely back and forth so that the image of.