. Biophysical science. Biophysics. 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 whi
. Biophysical science. Biophysics. 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 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 distan
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