2 : 2/ Light and the Eye 29 



phenomena of light which apply directly to vision. These include the 

 three avenues of approach outlined above, namely, geometrical optics, 

 electromagnetic waves, and the quantum theory of light. This is 

 followed by Section 3, on the anatomy of the human eye. The optical 

 properties of the eye considered as a thick lens, as well as visual defects, 

 are included in that section. Biophysicists have also been interested in 

 visual thresholds and in measurements of visual acuity ; these are dis- 

 cussed in the final section of this chapter. 



Many aspects of vision will be deferred to later chapters. For 

 example, color vision and the neural mechanisms making vision possible 

 are described in Chapter 7 which follows other chapters on the operation 

 of the nervous system. The properties of the retinal pigments which 

 absorb light are easier to understand following a study of enzymes. The 

 visual pigments are discussed in Chapter 19, Part D. Finally, Chapter 

 25, on information theory, contains a section which includes visual 

 information. 



2. Optics 



A. Geometrical Optics 



Many properties of lens and mirror systems can be treated by regarding 

 light as bundles of rays each of which moves at right angles to the wave- 

 front. This approach is utilized in this section in the discussion of the 

 properties of thick lenses. These properties are applied to the eye in 

 subsection B of Section 3. 



From the point of view of geometrical optics, the most important 

 property of a medium is the velocity at which light is propagated. In 

 free space, the velocity of light is usually designated by the symbol c, 

 and in cgs units, it has the value 



c = 3 x 10 10 cm/sec 



It is customary to specify the velocity v in any other medium by the 

 index of refraction, n. This is a dimensionless number defined by the 

 ratio 



* = - (1) 



v 



(Strictly speaking, n is always the index of refraction referred to the 

 velocity of light in free space. However, one may also use the relative 

 index of refraction n 12 between any two media, where n 12 is defined by 



■u = ?) (2) 



