CHAPTER 



THE EYE AND THE SENSE OF SIGHT 



131. By means of touch, taste, and smell the brain per- 

 ceives external objects through actual contact between 

 particles of matter from the objects perceived and the 

 human organism. But we need to be able to acquire 

 knowledge of the properties of objects at a distance from 

 ourselves. The sense of smell does indeed bring to us 

 limited information respecting some classes of objects at 

 no great distance; but it is by means of the eye and the 

 ear that we gain our most valuable knowledge of the uni- 

 verse, and through these that we enjoy the most refined 

 and elevated of all our pleasures. 



132. Vision. When rays of light fall upon a nervous 

 apparatus so made as to be affected by that stimulus, and 

 the impulse is carried by a nerve to the nerve center for 

 vision, there results the sensation of sight. Some animals 

 possess a simple arrangement for vision, consisting of only 

 three parts. Certain modified parts of the epidermis are 

 stimulated by the light, nerve fibers carry the impulse to 

 the nerve center, and light is perceived. Man, however, 

 is provided with organs of vision of elaborate and com- 

 plex structure. 



133. Light. All space is believed to be filled with an 

 extremely thin, perfectly elastic medium called ether, in 

 which atoms, molecules, and masses of matter are immersed 



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