THE EYE AS A SENSORY APPARATUS 241 



sence of sensation, while darkness causes a definite feeling of 

 "blackness." 



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 becomes deeper 

 until it passes into black ; and if the illumination be increased, the 

 color becomes " lighter" until it passes into white. Of all the 

 colors of the spectrum yellow most easily passes into white with 

 strong illumination. Black and white, with the grays which are 

 mixtures of the two, thus seem to stand apart from all the rest as 

 the fundamental visual sensations, and the others alone are in 

 common parlance named "colors." It has even been suggested 

 that the power of differentiating them in sensation has only lately 

 been acquired by man, and a certain amount of evidence has been 

 adduced from passages in the Iliad to prove that the Greeks in 

 Homer's time confused together colors that are very different to 

 most modern eyes; at any rate there seems to be no doubt that 

 the color sense can be greatly improved by practice ; women whose 

 mode of dress causes them to pay more attention to the matter, 

 have, as a general rule, a more acute color sense than men. 



Leaving aside black, white, gray, and the various browns 

 (which are only dark tints of other colors), we may enumerate 

 our color sensations as red, orange, yellow, green, blue, violet, 

 and purple; between each there are, however, numerous transi- 

 tion shades, as yellow-green, blue-green, etc., so that the number 

 which shall have definite names given to them is to a large extent 

 arbitrary. Of the above, all but purple are found in the spec- 

 trum given when sunlight is separated by a prism into its rays 

 of different refrangibility; rays of a certain wave-length cause in 

 us the feeling red; others yellow, and so on; for convenience we 



