240 THE HUMAN BODY 



regions as meaning left objects, and never get confused by the 

 inverted retinal image about which directly I know nothing. A 

 new-born child, even supposing it could use its muscles perfectly, 

 could not, except by mere chance, reach towards an object which 

 it saw; it would grasp at random, not yet having learnt that to 

 reach an object exciting a part of the retina above the fovea 

 needed movement of the hand towards a position in space below 

 the visual axis; but very soon it learns that things near its brow, 

 that is up, excite certain visual sensations, and objects below its 

 eyes others, and similarly with regard to right and left; in time 

 it learns to interpret 'retinal stimuli so as to localize accurately 

 the direction, with reference to its eyes, of outer objects, and 

 never thenceforth is puzzled by retinal inversion. 



Color Vision. Sunlight reflected from snow gives us a sensa- 

 tion 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 to 

 violet, through orange, yellow, green, blue-green, blue, and indigo. 

 The prism separates from one another light-rays of different 

 periods of oscillation 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, that of black ; 

 in this case the light-rays are so absorbed that but few are reflected 

 to the eye and the visual apparatus is left at rest. Physically 

 black represents nothing: it is a mere 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 dark- 

 ness 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 sensations. It is obvious, therefore, that 

 the sensation of blackness is not due to the -mere absence of lu- 

 minous stimuli, but to the unexcited 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 ab- 



