84: SPECIAL SENSES. 



vision. We can understand now the chromatic aberration is 

 practically corrected in the crystalline lens, when we remem- 

 ber that its various layers are of different consistence and of 

 different refractive power. 



Formation of Images in the Eye. 



It is only necessary to call to mind the general arrange- 

 ment of the different structures in the eye and to apply 

 the simple laws of refraction, to comprehend precisely how 

 images are formed upon the retina. 



The eye corresponds to a camera obscura. Its interior is 

 lined with a dark, pigmentary membrane, the choroid, the 

 function of which is to prevent the confusion of images by 

 internal reflection. The rays of light are admitted through a 

 circular opening, the pupil, the size of which is regulated by 

 the movements of the iris. The pupil is contracted when the 

 light striking the eye is intense, and is dilated as the amount 

 of light is diminished. In the accommodation of the eye, the 

 pupil is dilated for distant objects and contracted for near 

 objects ; for, in looking at near objects, the- aberrations of 

 sphericity and achromatism in the lens are more marked, and 

 the peripheral portion is cut off by the action of this movable 

 diaphragm, thus aiding the correction. The rays of light from 

 an object pass through the cornea, the aqueous humor, the 

 crystalline lens, and the vitreous humor, and are refracted 

 with so little spherical and chromatic aberration that the image 

 formed upon the retina is practically perfect. The layer of 

 rods and cones of the retina is the only portion of the eye en- 

 dowed directly with special sensibility, the impressions of 

 light being conveyed to the brain by the optic nerves. This 

 layer is situated next the choroid, but the other layers of the 

 retina, through w r hich the light passes to reach the rods and 

 cones, are perfectly transparent. 



It has been positively demonstrated that the rods and 

 cones are the only structures capable of directly receiving vis- 

 ual impressions, by the following interesting experiment, 



