536 VISION 



SECOND SECTION 



EXCITATION OF THE RETINA AND VISUAL SENSATIONS 



1. LIGHT RAYS 



Modern physics assumes the existence throughout all space of a rarefied 

 substance, the ether, which, although it has no weight, nevertheless obeys in 

 general the laws which govern the movements of molecules. The density of 

 the ether is so slight that it exercises no noticeable restraint on the movements 

 of the heavenly bodies, with the possible exception of the comets. 



Light is regarded as extremely rapid transverse (i. e., vertical to the 

 direction of propagation) vibrations of the ether, whidi are produced by the 

 luminous point and are propagated through the ether with very great velocity 

 (Huygens, 1678; Euler, Young, Fresnel). 



When sunlight enters a dark room through a small slit and then passes 

 through a glass prism, the small bundle of rays spreads out into a broad 

 band, called the solar spectrum, which is not now white, as the light orig- 

 inally was, but is of different colors arranged always in the same order: red, 

 orange, yellow, green, blue, indigo, and violet. Sunlight therefore consists 

 of rays which are refracted by the spectrum to different degrees, the red rays 

 being refracted least, the violet rays most. 



The difference in refrangibility of these rays is conditioned upon the dif- 

 ference in the rate of their propagation through solid and liquid media. They 

 are also distinguished by different vibration frequencies and consequently by 

 different wave lengths. The wave length (X) of extreme red rays is 760 

 millionths of a millimeter (/A/A), and of the extreme violet rays 397 /A/A. But 

 the solar spectrum is not limited to that which is visible to human eyes. It 

 contains also rays of greater wave length than 760 /A/A (ultra-red rays) and 

 rays of less wave length than 397 /A/A (ultra-violet rays). The former are 

 characterized especially by their thermal effects; the latter by their chemical 

 effects on certain silver salts. 



The ultra-red rays are not visible because, although they are transmitted 

 through the media of the eye, they do not stimulate the retina; while the 

 ultra-violet rays are invisible for just the opposite reason they are for the 

 most part absorbed by the media of the eye. When, as in the operation for 

 cataract, the lens is removed, the visible spectrum reaches down to A 313. 



Different sources of light contain the different rays in different quantity 

 e. g., the strontium light is red, the sodium light yellow. Accordingly, when 

 the light from such a flame is refracted by a prism we get, not a continuous 

 spectrum, but a spectrum which consists of more or less numerous distinct 

 luminous lines which are characteristic for the different chemical elements. 



The color of bodies which are not self-luminous depends upon the rays 

 which are reflected in greatest number from them, or, if the objects are 

 transparent, upon the rays which are transmitted by them. If, for example, 

 a surface is red, it is due to the fact that of all the rays which fall on that 

 surface, the red rays are thrown off in greatest number. Likewise a glass is 



