Page:Elementary Text-book of Physics (Anthony, 1897).djvu/457

§361] reflection. It is also the direction in which the angle of emergence equals the angle of incidence. The direction $$CE$$ corresponds to the minimum deviation for only one kind of light. If this be red light, the yellow will be more deviated, and the blue still more. To see these colors the eye must be higher up, or the drop lower down. If the eye remain stationary, other drops below will send to it the yellow and blue, and other colors of the spectrum. Since this effect depends only on the angle between the directions $$SA$$ and $$CE,$$ it is clear that a similar effect will be received by the eye at $$E$$ from all drops lying on the cone swept out by the revolution of the line $$CE$$ and all similar lines drawn to the drops above and below the drop $$O,$$ about an axis drawn through the sun and the eye, and hence parallel to $$SA.$$ This cone will trace out the primary rainbow having the red on the outer and the blue on the inner edge. The secondary bow, which is fainter, and appears outside the primary, is produced by two reflections and refractions as shown in Fig. 137.

361. The Solar Spectrum.—As has been stated (§ 357), solar light when refracted by a prism gives in general a continuous spectrum. Wollaston, in 1802, was the first to observe that when solar light is received upon a prism through a very narrow opening at a considerable distance, dark lines are seen crossing the otherwise continuous spectrum. Later, in 1814–15, Fraunhofer studied these lines, and mapped about 600 of them. That these may be well observed in the prismatic spectrum it is important that the apparatus should be so constructed as to avoid as far as possible spherical and chromatic aberrations. The slit must be very narrow, so that its images may overlap as little as possible. The most important condition for avoiding spherical aberration is that the waves reaching the prism should be plane waves, since all others are distorted by refraction at a plane surface. Fig. 138 shows the disposition of the essential parts of the apparatus known as the spectroscope. $$S$$ is the slit, which may be considered