416 ENERGY MAY BE PROPAGATED BY VIBRATIONS orange light, and lamps are used which give orange light free from blue light. Sodium-vapor lamps are satisfactory for this purpose. During the daytime, the orange light of the sun is absorbed by these blue filters, but the blue light is transmitted. Hues, Tints, and Shades Are Color Terms Used in Art. A hue is a color produced by one band of wave-length frequencies of the visible spectrum. A pigment hue is a pigment which reflects light of only one frequency or of a range of frequencies. The spectral colors, red, orange, yellow, blue, green, indigo, and violet, are hues. practiced eye may be able to recognize about 230 spectral hues. A tint is obtained by adding white to a hue; for example, pink is a tint obtained by adding white to red. A shade is obtained by adding black to a hue or a pure color. The relation of a hue to black or white is spoken of as value in art. The value is said to be lighter if white is added and darker if black is added. The various colors of the spectrum may be arranged in the order of their natural color value, yellow being the lightest, and blue-violet being the darkest ; this does not mean that black is present in the dark value or that white is present in the light value. Thus it is seen that value has a dual role in that it occurs in pure colors as well as in shades and tints. Values should be close when combining colors; for example, tints or light values are combined with neutral values as accents rather than with black, because black is too strong a contrast. If black were wanted as the accent, middle values would be used for the general scheme. Values are affected by the texture of the material on which they appear. The satin-backed crepe appears to be richer and darker on the satin side and duller and lighter on the crepe side. Intensity is a term used to designate the strength of a hue as compared with gray. Intensity is a term used to refer to the purity of a hue as contrasted with the same color to which has been added one or more other colors; for instance, pure red is more intense than red orange. The intensity of a hue of a given value may be decreased by adding neutral gray. A neutral gray is one which is produced by the admixture of complementary colors. To neutralize a color, one grays it by adding its complement. Colors May Also Be Produced by Interference. The brilliant colors of the plumage of peacocks and other birds are not due to colored pigments but to the interference phenomena which are produced by thin transparent films on the feathers. A Pearls, opals,
COLORS ARE PORTIONS OF VISIBLE SPECTRUM 417 soap bubbles, tempered steel, and oil films on water produce colors in the same way. Interference in light corresponds to interference in sound, which is discussed on p. 46L Colors May Also Be Produced by Scattering. The colors of the sky are due to the scattering of light by molecules of the air and particles floating in the air, just as the waves of the sea are turned aside and scattered by rocks that rise above the surface. The short waves of light that compose the blue end of the spectrum are more easily turned aside than the longer, red waves, just as ripples are turned aside by a rock over which the larger waves heave themselves and steadily advance. Thus a separation takes place and color is produced. For example, the smoke rising from a chimney looks blue against a dark background, especially if the smoke particles are quite small. If, on the other hand, the smoke is viewed against a bright background of luminous cloud or even against the sun itself, the color that comes through is brown or red. A red light carries better than a white light in a misty atmosphere because the water particles do not scatter the long, red rays so much as they scatter blue rays. Hence infrared photography is valuable in a hazy atmosphere. The setting sun and the rising full moon appear to be red because at the horizon the light has to travel a much greater distance through the atmosphere to reach the eye than it does when the sun or moon is directly overhead. The shorter blue waves are scattered, and only the longer rays toward the red end of the spectrum penetrate the great thickness of the atmosphere. The color of the sky in the morning and evening is largely due to scattering of light by dust and water vapor. When Krakatoa exploded and ejected huge quantities of very fine dust into the air half a centur>' ago, the colors of sunsets all over the world were strangely beautiful for many months afterward, because the dust drifted around the world and settled very slowly. Cloud particles reflect all colors of the spectrum, and therefore reflections from the clouds show masses of shining white; but when clouds lie between us and the sun, they may intercept nearly all of the light and appear black. Much of the blue color of the sea is due to the reflection from the sky. Under a leaden sky the sea looks gray. Near the shore the water is green because the fine sand in suspension reflects yellow, which is added to the blue light reflected from the sky.