[The eye, the optic system and its anomalies].

The eye is a perceptive system with extremely complex physiology, although its optical properties can be assimilated to those of spherical diopters. Various approximations make it possible to reduce the eyeball to a single convex diopter. With a normal eye the image of an object situated ad infinitum focuses on the retina. The normal eye is called emmetropic. Otherwise, the eye is called ametropic. Several types of ametropy exist. When the image focuses in front of the retina the eye is said to be myopic. When the image focuses behind the retina the eye is called hypermetropic (or hyperopic). When the image of an object differs according to various focusing axes, the eye is said to be astigmatic.