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 tinue adding orange in the larger disks until the difference cannot be detected between the small disk and the larger combined disks.

The standards may be combined in pairs, as has been indicated with the red and orange, to produce all the intermediate hues throughout the spectrum, but it must be remembered that these combinations are to be made by joining in pairs, colors adjacent in the spectrum, red and orange, orange and yellow, yellow and green, green and blue, blue and violet. We then shall have representations of all the spectrum colors, but there are still the colors between violet and red, known in nature and art as purples, which must be produced by. uniting the red and violet disks, thus completing a circuit of colors containing all the pure colors in nature.

In nature all colors are modified by light and shade, strong-light producing tints and shadows more or less deep forming-shades.

These effects are imitated on the color wheel by the use of a white disk combined with a disk of a standard color for tints and a black disk for shades, and can be tested in the same order as indicated for the hues, by combining each standard disk with a white or a black disk in varying proportions. It will be noticed early in disk experiments that a very small amount of white produces a decided effect in the tone of a color while a comparatively large amount of black is necessary to produce a marked change. As this is exactly the reverse of the effects of white and black pigments it is always a subject of remark. In pigments these effects are imitated by the mixture of white with a color to produce tints, and black for shades, or more generally instead of black some dark natural pigment approaching the hue of the color, may be preferred because a black pigment will too often impart an unexpected and undesirable hue to the color. As for example, in making shades of red some natural brown pigment is better than black, and so various dark browns and grays are used for different colors.