110 



BIOPHYSICAL CHARACTERISTICS OF THE EYE 



sharpness, and that the lines also seem curved. This imperfect focusing 

 results from the fact that this lens reproduces straight lines of a flat 

 object as curved lines concave toward the lens. A diaphragm placed 

 behind such a lens, leaving only the central area of the lens in use, will 

 block out the peripheral field and allow only the axial rays to pass 

 through the stop to form the image. These conditions are illustrated 



Fig. III-6. 



in Fig. Ill— 6, which shows the refracted path of the rays that pass 

 through different zones of the lens. The outer zones are focused at c; 

 the more axial zones are focused further out at b. The aberration is 

 measured by the distance cb. A stop placed behind the lens as in 

 Fig. III-6 reduces the aberration and sharpens the focus. 



Contraction of the pupil causes a reduction in intensity of the light 

 and limits the beam of light to the central zones of the lens. The 

 accompanying reduction in axial spherical aberration improves the 

 definition of the image. 



Spherical Aberration of the Eye 



Since the cornea and the lens are nearly spherical surfaces, it might be 

 expected that spherical aberration must be present. The crystalline 

 lens, however, has a structure essentially different from that found in 

 optical instruments. Owing to the graduated changes of index of 

 refraction, rays passing through the central zone of the lens are refracted 

 to a greater extent than the more peripheral rays, so that the rays 

 marked b in Fig. Ill— 6 are more nearly superimposed on the rays c. In 

 addition the more peripheral parts of the lens are flattened ; therefore, 

 the lens deviates the rays less than those passing through this area of a 

 spherical lens. 



Chromatic Aberration of the Eye 



In discussing the simple geometrical formation of images by lenses, 

 it is always assumed that light is monochromatic. As a matter of 



