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 neutral gray, which is a white in shadow or under a low degree of illumination. If to such a gray a very small amount of color is added, as orange for example, by the introduction of an orange disk, this neutral gray becomes an orange gray, but unless the amount is considerable it can not be detected as an orange, but the gray may be termed a warm gray, denoting that it is affected by some one of the colors near the red end of the spectrum. If blue instead of orange is added to the neutral gray, a cool gray is produced. When green is added to a gray the result can not fairly be called either warm or cool, and hence we have termed it a green gray. According to this plan we have four classes of grays, Neutral, Warm, Cool and Green grays. As there may be many tones of each, and many intermediate combinations from red to green, or green to blue, the number of grays in nature is infinite, but these four classes with two tones of each in the papers form what may be called standards or stations from which to think of the grays, the same as the six standards in the spectrum constitute points from which to think of pure colors.

A careful consideration of the foregoing pages, accompanied with a color wheel or even a color top, can hardly fail to give a student who will make the experiments a clear idea of the use of the disks in the system of color education in which they form such an important feature, and therefore the old theory of three primaries, red, yellow and blue, and all that it leads to can be very intelligently considered and tested by them in the experiments which follow.

This old theory briefly restated is as follows: It is said "there are in nature three primary colors, red, yellow and blue; and by the mixture of these primary colors in pairs, orange, green and violet may be made." In fact leading educators have said that "in the solar spectrum, which is nature's chart of colors, the principal colors are red, orange, yellow, green, blue and violet; of these red, yellow and blue are primaries from which may be made the secondaries, orange, green and violet."