ABSTRACT
The first serious studies were carried out by Helmholtz, as described in his book Optik. Eye studies continued in the 19th century with the pioneering work by Gullstrand. A diagram of the human eye is presented in Fig. 13.1 and its main optical constants are listed in Table 13.1. The most important optical components of the eye are:
The cornea-This is the front transparent tissue in the eye. Its normal ideal shape is nearly spherical, with a dioptric power of about 43 diopters. Any deviation from its ideal shape produces refractive errors. If it takes a toroidal shape, with different curvatures along two mutually perpendicular diameters, corneal astigmatism appears. The astigmatism is said to be with the rule if the curvature in the vertical diameter is larger than in the horizontal diameter and against the rule otherwise. A small protuberance and thinning at the center makes the cornea to have an almost conic shape, in a defect called keratoconus. These errors are measured with an ophthalmeter or a corneal topographer.
