[Illustration:  FIG. 88.—­Violet and green give blue.  Green, blue, and red give white.]

133.  Colors not as they Seem—­Compound Colors.  If one half of a cardboard disk (Fig. 88) is painted green, and the other half violet, and the disk is slipped upon a toy top, and spun rapidly, the rotating disk will appear blue; if red and green are used in the same way instead of green and violet, the rotating disk will appear yellow.  A combination of red and yellow will give orange.  The colors formed in this way do not appear to the eye different from the spectrum colors, but they are actually very different.  The spectrum colors, as we saw in the preceding Section, are pure, simple colors, while the colors formed from the rotating disk are in reality compounded of several totally different rays, although in appearance the resulting colors are pure and simple.

If it were not that colors can be compounded, we should be limited in hue and shade to the seven spectral colors; the wealth and beauty of color in nature, art, and commerce would be unknown; the flowers with their thousands of hues would have a poverty of color undreamed of; art would lose its magenta, its lilac, its olive, its lavender, and would have to work its wonders with the spectral colors alone.  By compounding various colors in different proportions, new colors can be formed to give freshness and variety.  If one third of the rotating disk is painted blue, and the remainder white, the result is lavender; if fifteen parts of white, four parts of red, and one part of blue are arranged on the disk, the result is lilac.  Olive is obtained from a combination of two parts green, one part red, and one part black; and the soft rich shades of brown are all due to different mixtures of black, red, orange, or yellow.

134.  The Essential Colors.  Strange and unexpected facts await us at every turn in science!  If the rotating cardboard disk (Fig. 88) is painted one third red, one third green, and one third blue, the resulting color is white.  While the mixture of the spectral colors produces white, it is not necessary to have all of the spectral colors in order to obtain white; because a mixture of the following colors alone, red, green, and blue, will give white.  Moreover, by the mixture of these three colors in proper proportions, any color of the spectrum, such as yellow or indigo or orange, may be obtained.  The three spectral colors, red, green, and blue, are called primary or essential hues, because all known tints of color may be produced by the careful blending of blue, green, and red in the proper proportions; for example, purple is obtained by the blending of red and blue, and orange by the blending of red and yellow.