520 THE HUMAN BODY. 



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 sur- 

 face weakens, the shade becomes deeper until it passes into 

 black; and if the illumination is 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 enum- 

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

 violet and purple; between each there are, however, numer- 

 ous transition shades, as yellow-green, bine-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 spectrum given when sunlight is 

 separated by a prism into its rays of different refrangibility; 

 rays of a certain wave-length or period of oscillation cause in 

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



