1. An ophthalmoscopic instrument comprising objective lens means having an optical axis and adapted to be positioned to confront a patient's eye at a working distance, said patient's eye having a cornea with a center of curvature, an observing optical system adapted to focus a light beam reflected from the eye and passing through the objective lens means on an image plane, means for detecting that the eye is properly positioned with respect to the objective lens means, said detecting means including a single target mark, a target mark image projection system adapted to project said target mark image by way of at least two light beams crossing each other on the optical axis of the objective lens means so that a target image is formed by each of the beams at the center of curvature of the cornea of the eye when the eye is properly positioned with respect to the objective lens means, a target mark image plane wherein target mark images are formed by mirror reflection, at the cornea, of the light beams projected through the target mark projection system when the patient's eye is properly positioned, a target mark image observation system for observing the target mark images on the target mark image plan in superposition with a reference mark, whereby said target mark images on the target mark image plane are formed in superposition with said reference mark at predetermined positions when the eye is properly positioned with respect to the objective lens means.