a method and a device for making the distance between a human eye and an eye - examining instrument coincide with a preset distance . a miniature light source disposed on the image end of an objective lens of the instrument illuminates the eye . the light specularly reflected by the surface of a cornea is converged . the miniature light source is positioned to have the converging point lie on the surface of an image of the fundus formed by the objective lens . accordingly if the distance between the human eye and the instrument coincides with a preset distance , the image of the light source on the image surface of the objective lens becomes sharp . an operator looking into a view finder correctly adjusts the position of the instrument by moving the latter until the image of the light source becomes sharp .