[Johannes Kepler's contributions to ophthalmologic optics]

Until the beginning of the 17th century it was held that an image is formed in the eye on the anterior surface of the crystalline lens. Ophthalmological optics as a scientific discipline only began with a discovery made by Johannes Kepler. Without performing new experiments, and solely by application of the laws of light refraction, he analyzed the path of light through the eye and demonstrated that an image is formed on the retina and that it is inverted. Acceptance of this discovery was impeded by contemporary prejudices which could imagine nothing but an upright image in the eye, even though this attitude could not explain certain phenomena. Kepler's discovery of the path of light in the eye made it possible to explain the following physical phenomena: central visual acuity, visual field, dark adaptation, and errors of refraction. Physiological diplopia and the mechanism of accommodation were discovered later. The law stating that the intensity of light decreases with the square of distance was also formulated by Johannes Kepler; this law, too, could only be demonstrated after the path of light through the eye had been discovered. In recent years the Keplerian telescope has assumed a practical significance in ophthalmological optics. As a reading aid for individuals with impaired vision it offers a significantly higher magnification than any other optical visual aid.