36 



Light and the Eye /2 : 3 



disappear when its image falls on the blind spot. In Figure 4, the x , 

 the . , and the : disappear at different distances. 



Within the eye there are additional, optically important structures. 



One of these is the iris, which acts as a light 

 diaphragm. In bright light, the iris has a min- 

 imum opening. This is desirable for several 

 reasons. A smaller opening means fewer light 

 photons enter the eye, thereby decreasing the 

 "overloading" of the retinal system. In addi- 

 tion, it improves the validity of the approxi- 

 mation which is made in the discussion of 

 spherical lenses, namely, that just a small 

 section of a sphere is used. 2 Thus, a small iris 

 opening limits such distortions as spherical 

 aberration, field curvature, and coma asso- 

 ciated with finite sections of spheres. Finally, 

 a small iris opening increases the depth of 

 focus. The reason for this can be seen from a 

 simple ray diagram, such as is shown in 

 Figure 5. At night, maximum acuity and 

 depth of focus are less important than maxi- 

 mum sensitivity. At this time the iris is opened 

 to its widest. 



Another optically significant structure within 

 the eye is the crystalline lens. In spite of its name, this is actually a cellular 

 structure. The rear face is curved more sharply than the front. The 

 eye accommodates to objects at different distances by changing the cur- 

 vature of the front face of this lens. When the object is farther away, a 

 weaker lens is needed to focus the image on the retina than when the 

 object is closer. Hence, for more distant objects, the lens must be 

 flatter, whereas, for closer objects, it must become more curved. 



The shape of the crystalline lens is controlled by a ring of muscles 

 surrounding the lens. These are called the ciliary muscles. Most 

 physiologists believe that the lens is normally held in a strained position 

 by the ciliary fibers. These fibers hold the lens in a flattened condition 

 suitable for viewing distant objects. When the ciliary muscle contracts, 

 it moves the base of the fibers forward permitting the lens to relax into 

 a more curved shape. When the muscle relaxes, the lens is again placed 

 under tension. 



The space between the lens and the retina is filled with the vitreous 

 humor, a jelly-like mass of material traversed by fibrils. Staining 



Figure 4. Pattern to ob- 

 serve the blind spot in the 

 eye. Fixate the right eye 

 on the large dot and bring 

 the face very close to the 

 figure. Now slowly move 

 the face away while keep- 

 ing the right eye fixated on 

 the large dot. The other 

 symbols will disappear and 

 then reappear as their 

 images cross the blind spot 

 on the retina. 



2 See Appendix B. 



