xvi ii THE SPECIAL SENSES AND SENSATIONS 265 



drugs, and many other causes. A person may suffer 

 from defective hearing without being aware of it at all. 

 It is important to test the ears of children by some 

 simple method, as seeing how far they can hear the 

 ticking of a watch with each ear separately. 



Blindfold a person and cover one ear, and then hold a ticking 

 watch close to the other ear. Carry it away slowly and determine 

 how far it can be heard. Repeat for the other ear. Are the two 

 ears equally sensitive to sound? 



182. The Sense of Sight. Sight is probably the most 

 important sense we have. The beautiful colors in 

 nature and art would be entirely unknown, and life 

 would lose many of its pleasures, if we could not see. 

 Seeing is even more wonderful than hearing. We 

 learned that the beautiful music of an orchestra and 

 the sweet voice of a friend are simply vibrations of air. 

 All objects that we can see give forth vibrations to the 

 ether. The beautiful color of the rose is simply ether 

 vibrations that can act upon the delicate eye in such a 

 way as to cause sensations of sight. The vibrations of 

 ether differ from each other and thus give rise to the 

 different colors of the rainbow and their many shades 

 and tints. We shall now study the eye and learn how 

 we see. 



183. The Eyeball. The eyeball is spherical in shape, 

 measuring about one inch from side to side and a little 

 less from front to back. It is set in a bony cavity 

 called the orbit, which is lined by a mass of connective 

 tissue and fat. The back two thirds of the eyeball 

 is inclosed in a capsule of serous membrane like the 



