684 THE POPULAR SCIENCE MONTHLY. 



simultaneously or at very close intervals, produces the impression of 

 white. Starting from a preconceived analogy with the notes of the 

 gamut, Newton divided the solar spectrum, or the image obtained by 

 decomposing white light with a refracting prism, into seven different 

 colors. This division is really arbitrary, for the colors pass from one 

 to another by insensible transitions, and each of them may be charac- 

 terized either by the degree of its refraction in the prism, or by the 

 length of the undulations to which it corresponds. 



When we collect a part of the spectral rays in one point, we obtain 

 either one of the primitive colors in a greater or less state of purity, 

 or a new tint. If we divide the spectrum arbitrarily into two parts 

 and collect the rays of these parts separately, we obtain two distinct 

 colors, the superposition of one of which on the other gives white light. 

 The experiment can be performed with an ordinary spectrum divided 

 arbitrarily into two parts ; and it is effected, we might say naturally, 

 in the phenomena of rotatory polarization in which the most brilliant 

 hues are shown. 



We mention these properties to deduce two conclusions from them. 

 We remark, first, that the mixture of simple or homogeneous lights in 

 any proportion always produces upon the eye a single impression, that 

 of one color. While the ear can distinguish all the notes that go to 

 make up a harmony, the eye can grasp only one color, without being 

 able to distinguish whether it is really simple, or is formed of different 

 lights. 



In the second place, the mixture of colors provokes only one new 

 impression, that of purple, for example, which we may obtain by mix- 

 ing red and violet, while the varieties of rose are nothing but mixtures 

 of purple and white. White may be produced by two simple colors 

 alone, as by red and green ; more generally, if we isolate three suita- 

 bly chosen colors in the spectrum, such as particular shades of red, 

 green, and violet, we may, by mixing them in different proportions, 

 imitate the impressions produced by all the colors. Artificial colors, 

 formed, for example, of rays selected from the spectrum, may be sim- 

 ple or compound, without the eye being informed of the difference, 

 except, perhaps, when they have a shade of purple or of rose, for we 

 know then that those colors do not exist in a simple state. 



The same is the case with the colors of nature, or of industry. An 

 object appears colored to us because it sends us only a part of the 

 light it borrows from the general illumination. The sorting out is 

 made either by transmission, as in colored glasses, or by reflection, as 

 in the case of the metals, or by diffraction, as in the wings of some 

 butterflies, or in the coronas which we sometimes perceive around the 

 moon ; the portion of the light that does not reach the eye having 

 been absorbed or sent off in a different direction. Leaving out of the 

 account the effects of fluorescence, we perceive that objects do not 

 have colors of themselves, but simply borrow from the general light- 



