THE SPECIAL SENSES. 19? 



the fact that the blind are commonly 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 incalcu- 

 lable, 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 undula- 

 tory theory. This theory supposes that there exists an 

 intangible, elastic medium, which fills all space, and pene- 

 trates all transparent substances, and which is thrown into 

 exceedingly rapid undulations or waves, by the sun and 

 every other luminous body; the undulations being propa- 

 gated 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 

 sensation 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 expansion of the optic nerve. It is sensitive 



47. Sight, unlike the other senses ? In the case of the stars ? 



48. The undulatory theory oflight? What does the theory suppose ? 



49. The sensation of light V Optic nerve ? 



