Fig. 40.
If a beam of light be transmitted through a prism of glass the rays are decomposed, and what is known as a spectrum is formed (Fig. 40). The most generally observed spectrum is the rainbow. When the light from a flame in which is burning some suitable substance be transmitted through the prism, the color which predominates in the flame will predominate in its spectrum. The combination of a prism and tubes for observing these effects is a spectroscope (Fig. 41). The short fat spark from the Ruhmkorff coil is most useful in this work. The electrodes are provided with a portion of the substance to be examined, and the spark is passed and viewed through the spectroscope.
Fig. 41.
The spectroscope is shown in connection with the coil in Fig. 41. A is the aperture in the screen through which the rays from the metal burning at the discharger balls D D passes. The lens at L is used to view these rays after they have been decomposed by the prism P, which, as well as the lens, can be rotated. I is the coil, P P the primary and S S the secondary wires, C being a condenser bridged across the circuit.
The screen should be pierced by a very narrow aperture, A, and be placed at a considerable distance from the prism P, that the rays issuing through the aperture may not strike the prism until they have widely diverged and become separated from each other. The aperture is practically formed of perfectly parallel knife edges, forming a slit not exceeding one hundredth of an inch in width.
The colored spaces in the solar spectrum do not occupy an equal extent of area; the violet is the most extended, the orange the least. The proportion is in three hundred parts: Violet, 80; green, 60; yellow, 48; red, 45; indigo, 40; orange, 27.
The solar rays exhibit on careful examination dark lines crossing the spectrum at right angles to the order of the colors, and always occupying the same relative positions. These are called Fraunhofer's lines.
If, however, the spectra of metals, gases, and other elements be examined they will be found to present certain characteristic bright lines, the body of the spectrum being often feeble or entirely dark. The spectrum of hydrogen gives two very bright lines of red and orange.