=399. Color.=--Much of the pleasure experienced in gazing at beautiful objects is due to the _color_ shown by them. The blue sky, the green grass, and the varied tints of flowers, and of the rainbow all excite our admiration The study of color begins naturally with the production of the _spectrum_, the many-colored image upon a screen produced by passing a beam of light through a prism. The spectrum is best shown when the light enters by a narrow slit (Fig. 400). The spectrum was first produced by Sir Isaac Newton in 1675 by the means just described. The names usually given to the more prominent colors of the spectrum are violet, indigo, blue, green, yellow, orange, and red. The initials of these names, combined, spell _vibgyor_, a word without meaning except to assist in remembering the order of the colors in a spectrum. If the light that has passed through a prism is sent through a second prism placed in reverse position (see Fig. 401), the light passing through both prisms is found to be white. This experiment _indicates that white light is composed of light of all colors_. [Illustration: FIG. 400.--Formation of the spectrum by a prism.] [Illustration: FIG. 401.--The colors of the spectrum recombine to form white light.] =400. Dispersion.=--The separation of the colors by a prism is called dispersion. In experimenting to find a reason for dispersion, it has been learned that lights of different colors are of different wave lengths. Color in light is therefore analogous to pitch in sound. We hear through many octaves, but we see through about one octave. That is, the shortest visible waves of violet light are about 0.000038 cm. in length while the longest visible red rays are 0.000076 cm., or the longest visible light waves are about twice the length of the shortest visible ones. It appears from the evidence of experiments upon dispersion that _light waves of different lengths are refracted differently_. This causes the images formed by refraction through simple glass lenses to be fringed with color and to lose some of their sharpness and definiteness of outline, since the violet light is brought to a focus sooner than the red. (See Fig. 402.) This seriously affects the value of such lenses for optical purposes. Fortunately it is found that _different kinds of glass have a different rate of dispersion for the same amount of refraction_. [Illustration: FIG. 402.--Violet light comes to a focus sooner than red.]