THE EYE AND THE PHYSIOLOGY OF VISION. 559 



formation of a spectrum is the result of such an unequal 

 bending, the red rays being bent least, the violet rays most. 

 Evidently, therefore, since white light is composed of the 

 seven fundamental colors, the violet rays being bent more 

 than the red rays will come to a focus sooner, and in this 

 way there will be produced a number of foci corresponding 

 to the number of different colors, and objects will appear 

 violet, greenish, or red, according as the violet, green, or 

 red focus falls on the retina. 



Fig. 175. DIAGRAM TO ILLUSTRATE THE CHROMATIC ABERRATION OF THE EYE. 

 r, r', red rays; t?, ', violet rays; V, violet focus; B, red focus; m, n, position of image 

 with a red border; p, s, position of image with a violet border. 



This chromatic aberration is most marked near the edge 

 of a lens, and a peep through most opera-glasses shows this 

 defect very plainly. While the center of the field is plain 

 white, the edge of the field is tinted with some color due to 

 chromatic aberration. This defect is reduced to an almost 

 imperceptible minimum in the human eye by the exclusion 

 of the light through the edge of the lens. 



7. Muscae-Volitantes. While the humors of the eye 

 are practically quite transparent there occur not infrequently 

 small opaque particles floating in them, the shadow of which 

 is by the light entering the pupil thrown against the retina 

 and there perceived as a more or less black object, which by 

 the mind is projected out into space. These are the flying 

 motes. These opaque particles are remnants of embryonic 

 blood-vessels in the humor. 



8. Presbyopia. As the name indicates, this is a disease 

 which arises usually in advancing age. The ability to focus 



