CHAPTER XXII 

 THE EYE 



SIGHT is considered the most important of the sensations 

 It is the chief means of bringing the body into proper re- 

 lations with its surroundings and, even more than the sen- 

 sation of hearing, is an avenue for the reception of ideas. 

 The sense organs for the production of sight are the eyes ; 

 the external stimulus is 



Light. Light, like sound, consists of certain vibrating 

 movements, or waves. They differ from sound waves, 

 however, in form, velocity, and in method of origin and 

 transmission. Light waves are able to pass through a 

 vacuum, thus showing that they are not dependent upon 

 air for their transmission. They are supposed to be trans- 

 mitted by what the physicist calls ether a highly elastic 

 and exceedingly thin substance which fills all space and 

 penetrates all matter. As a rule, light waves originate in 

 bodies that are highly heated, being started by the vibra- 

 tions of the minute particles of matter. 



Light is influenced in its movements by various conditions. In a 

 substance of uniform df E*JJ|J it moves with an unchanging velocity and 

 in a straight fine. If it enters a less dense, or rarer, substance, its ve- 

 locity increases : if one more dense, its Telocity diminishes ; and if it 

 enters either the rarer or denser substance in any direction other than 

 perpendicularly, it is bent out of its coarse, or refracted. If it strikes 

 jgjJH-U a body tying; in its coarse, it may be thrown off (reflected), or 

 it may enter the body and either be passed on through (transmitted) 

 or absorbed (Fig. 157). Light which is absorbed is transformed into 

 beat 



