COLOR VISION. 519 



near its brow, that is up, excite certain visual sensations, and 

 objects below its eyes others, and learns to interpret retinal 

 stimuli so as to localize accurately the direction, with ref- 

 erence to its eyes, of outer objects, and never thenceforth 

 gets puzzled by retinal inversion. 



Color Vision. Sunlight reflected from snow gives us 

 a sensation which we call white. The same light sent 

 through a prism and reflected from a white surface excites 

 in us no white sensation but a number of color sensations, 

 gradating insensibly from red tr> viol e| f , +.V>rm|grli nvgrvo-Q 



blue, and indigo. The prism 



separates iiv n one another light-rays of different periods of 

 oscillation (p. 494) and each ray excites in us a colored 

 visual sensation, while all mixed together, as in sunlight, 

 they arouse the entirely different sensation of white. If 

 the light fall on a piece of black velvet we get still another 

 sensation, thaljiLJilack^ in tMs^ase_theJ.[ght-rays are so 



absorbed that but few are reflected to the eve and the vis- 

 -^^ ' *- 



nal apparatus is left at rest. Physically black repre- 

 sents nothing: it is"lTniere~zero the absence of ethereal 

 vibrations; but, in consciousness, it is as definite a sensation 

 as white, red, or any other color. We do not feel blackness 

 or darkness except over the region of the possible visual 

 field of our eyes. In a perfectly dark room we only feel the 

 darkness in front of our eyes, and in the light there is no such 

 sensation associated with the back of our heads or the palms 

 of our hands, though through these we get no visual sensa- 

 tions. It is obvious, therefore, that the sensation of blackness 

 is not due to the mere absence of luminous stimuli but to the 

 nnexcited state of the retinas, which are alone capable of 

 being excited by such stimuli when present. This fact is 

 a very remarkable one, and is not paralleled in any other 

 sense. Physically, complete stillness is to the ear what 

 darkness is to the eye; but silence impresses itself on us 

 as the absence of sensation, while darkness causes a definite 

 feeling of " blackness." 



Young's Theory of Color Vision. Our color sensations 

 insensibly fade into one another; starting with black we can 

 insensibly pass through lighter and lighter shades of gray 



