542 THE HUMAN BODY. 



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 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 sensations. It is obvious, therefore, that the sensation 

 of blackness is not due to the mere absence of luminous 

 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 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 

 to white: or beginning with green through darker and darker 

 shades of it to black or through lighter and lighter to white: 

 or beginning with red we can by imperceptible steps pass to 

 orange, from that to yellow and so on to the end of the solar 

 spectrum: and from the violet, through purple and carmine, 

 we may get back again to red. Black and white appear to be 

 fundamental color sensations mixed up with all the rest: we 

 never imagine a color but as light or dark, that is as more or 

 less near white or black; and it is found that as the light 

 thrown on any given colored surface weakens, the shade be- 

 comes deeper until it passes into black; and if the illumina- 

 tion be increased, the color becomes "lighter" until it passes 



