138	THE DIAMOND
	gether with others which result, in their effect upon the eye, from modifications by combination with each other. These colors appear in the spectroscope to the eye, in horizontal bands of variable width according to the media through which the light passes, as violet, indigo, blue, green, yellow, orange and red. Now the different elements of the various objects we see, when white light falls upon them, absorb some of its constituent rays and return some to the eye separated from the others, thereby producing the various sensations of color. There are several reasons, however, why persons differ in their judgment of them. Practically every stone has qualities which would produce a definite degree of color to the eye under the same conditions. But exactly the same conditions always are almost impossible, for the varia­tion of position when the stones are placed side by side is sometimes sufficient to affect the light vibrations and therefore the color appearance to the eye, in favor of one of them. Again, the eyes which see, vary. A ray of pure white light passed through a prism divides, on the screen, into the spectroscopic bands, which merge one into the other in a definite unvarying gradation, but no two persons would draw the dividing lines between them in the same places. One sees more yellow and less green and the others vice versa. Beyond this, it is being found that many people are absolutely blind to some colors. There is of course a normal average perception, but many are not up to that average, and of those above it, few have trained the faculty, under the distractions of a broken surface of sharply reflective and refractive material, suf­ficiently to see clearly the exact color of it. The sensa-