THE SPECIAL SENSES 273 



in support of their position the fact that the blind are com- 

 monly cheerful and gay, while the deaf are inclined to be 

 morose and melancholy. In respect to the relative capacity 

 for receiving education in the deaf and blind, it is found that 

 the former learn, more quickly, but their attainments are not 

 profound ; while the blind acquire more slowly ^ but are able 

 to study more thoroughly. 



47. Light The Optic Nerve. Unlike the senses previously 

 considered touch, taste, and smell sight does not bring us 

 into immediate contact with the bodies that are examined ; but, 

 by it, we perceive the existence and qualities of objects that are 

 at a greater or less distance from us. In the case of the stars, 

 the distance is incalculable, while the book we read is removed 

 but a few inches. Light is the agent which gives to this sense 

 its wide range. The nature of this mysterious force is not 

 known, and it is not here to be discussed, since its study 

 belongs more properly to the province of natural philosophy. 



48. It is sufficient, in this connection, to state that the theory 

 of light now generally accepted, and which best explains the 

 facts of optics, is that known as the undulatory theory. This 

 theory supposes that there exists an intangible, elastic medium, 

 which fills all space, and penetrates all transparent substances, 

 and which is thrown into exceedingly rapid undulations or 

 waves, by the sun and every other luminous body the undu- 

 lations being propagated with extreme rapidity, and moving 

 not less than 186,000 miles in a second. 



49. These waves are thought to produce in the eye the sen- 

 sation of light, in the same manner as the sonorous vibrations 

 of the air produce in the ear the sensation of sound. That 

 part of the eye which is sensitive to these waves is the expan- 

 sion of the optic nerve. It is sensitive to no other impression 

 than that of light, and it is the only nerve which is acted upon 

 by this agent. The optic nerve, also called the " second pair " 

 of cranial nerves, is the means of communication between the 

 eye and the brain. 



47. Sight unlike the other senses ? In the cas^Wl^st^? 



48. The undulatory theory of light ? Wlisit does UN- theory suppose ? 



49. The sensation of light ? Optic nerve ? 



