34 Light and the Eye /2 : 3 



and of quantum mechanics is the only one in which these topics are 

 discussed. The reader with a background in biology, or even an 

 undergraduate physics major, may feel that this topic of quantum 

 mechanics has been introduced too lightly, but only the concepts of 

 quantum mechanics which are needed for a discussion of vision have 

 been included above. 



Quantum mechanics is necessary for an understanding of character- 

 istic spectra. Accordingly, quantum theory is discussed more thoroughly 

 in Chapter 27. Even there, the author must make several statements 

 which are foreign to everyday experience and certainly are not proved 

 in this text. It is hoped that, in spite of this, the reader will at least gain 

 a feeling of what quantum mechanics is and how it is used, even though 

 he may be completely unable to manipulate it. 



3. Anatomy of the Eye 



A. Gross Anatomy 



The gross anatomies of all the vertebrate eyes are very similar. For 

 simplicity, numerical values will be given only for the human eye. The 

 human eyeball is roughly a sphere approximately 2.4 cm in diameter. 

 It is supported in a special socket in the cranium. The orientation of 

 the eyeball is controlled by six sets of muscles. These rotate the eyeball 

 quite freely because the socket is well lubricated. The muscles are con- 

 trolled by three pairs of nerves. The relative tensions in the muscles 

 are signals which might be used by the brain to determine the location 

 of the object viewed. 1 Many binocular judgments of distance, size, and 

 orientation could be "computed" by the central nervous system from 

 data on the relative tensions of these muscles. 



The external covering of the eyeball is made up of three spherical 

 layers, as shown in Figure 3. The outermost is the sclera. It is a 

 white fibrous coat commonly called the "white of the eye." At the 

 very front portion of the eye, the sclera leads into the cornea, a clear 

 transparent structure which admits light into the eye. The human 

 cornea is about 12 mm in diameter and has a radius of curvature of 

 about 8 mm. A major part of the refractive power occurs at the cornea. 



On top of the sclera is another thin layer called the choroid layer. It 

 contains the blood vessels and a pigmented substance. The choroid 

 layer does not continue all the way around to the cornea, as is shown in 

 Figure 3. 



1 The evidence as to whether or not this information is actually used by humans 

 is quite controversial. 



