2 : 3/ Light and the Eye 



39 



average human eye from the second principal point H' to the posterior 

 focal point +F is 2.2 cm. Thus, the eye has a strength of about 48 

 diopters. 



If the eye is stronger than this, images of distant objects will be 

 focused in front of the retina. Such an eye is called near-sighted or 

 myopic because near objects will be focused on the retina. This ocular 

 defect can be corrected by placing a negative (diverging) lens in front 

 of the eye. "Normal" vision is the ability to focus on the retina images 

 of objects more than 25 cm away. 



If the refractive power of the eye is too weak, the image will be formed 

 behind the retina, and positive lenses are needed for correction. Such 

 eyes are called hyperopic or far-sighted. By and large, it is not possible to 



design a corrective positive lens for objects at 

 all distances and so bifocals or trifocals are 

 necessary. 



Another frequent defect, which can be 

 corrected by glasses, is called astigmatism. 

 This defect consists of having different focal 

 lengths for lines in different directions. A 

 so-called normal person would see all the lines 

 of a fan chart, Figure 7, as equally black, 

 whereas one with astigmatism will see lines 

 in one meridian darker than those in the 

 meridian at right angles. Astigmatism is due 

 to the fact that some of the refractive surfaces 

 of the eye, especially the cornea, are not 

 spherical but have different curvatures in two meridians. 



To recapitulate, the eye lends itself to a description in the terms of 

 geometrical optics. The eye is a system of spherical surfaces separated 

 by media of different indices of refraction. Optically, it can be des- 

 cribed in terms of six cardinal points. The common defects easily 

 corrected by glasses can also be described in the language of geometrical 

 optics. 



Figure 7. Pattern for observ- 

 ing astigmatism. 



C. Histology of the Eye 



Each gross structure of the eye can be described on a microscopic scale. 

 This is the role of histology. The evidence from histology, in turn, 

 forms part of the basis of the biophysics of vision. Without a knowledge of 

 the histology of the retina, there can be no neural interpretation of vision, 

 such as is discussed in Chapter 7. Likewise, the parts of the eye, referred 

 to in subsection B, can be described in terms of their histological structures. 



