The coincidence optometer

The power of the required spectacle lens may be determined objectively by the ophthalmoscopic observation of the retinal image of a target, the position of which can be adjusted relative to an optometer lens to bring it to the conjugate of the retina. In the past the method has depended upon the observer's estimation of focus, or of parallax between the retinal image and the target which is in the field of the observing system, or of coincidence between the retinal image and an index line in the observing system. This paper describes the principle of an instrument in which the adjustments for both refraction measurement and meridian location of an astigmatic eye are made by a coincidence setting between two parts of the retinal image. By this means the sensitivity of the method is increased by more than 100 per cent.