Fraunhofer had noticed that the yellow spectrum line from common salt, when fed into a spirit lamp, was identical in position with the D-line of the solar spectrum. But though the formation of these discontinuous spectra from various salts in a flame was generally known, it was not until 1859 that the presence of the Fraunhofer lines in the solar spectrum was clearly explained by Kirchhoff, who deduced the following law: "The relation between the powers of emission and the powers of absorption for rays of the same wave-length is constant for all bodies at the same temperature." Thus the particles of a substance under the excitement of some outside force are thrown into a state of vibration which is dependent upon the chemical nature of the substance itself. This vibratory motion gives rise to waves in the ether and we have the phenomenon of emission. Again the particles of a substance are most responsive to these same characteristic vibrations and will absorb them whenever present, just as, by analogy, the strings of a piano pick up sound waves of the exact period in which they vibrate when these waves are set in motion by other musical instruments in the neighborhood. Kirchhoff explained the solar spectrum as one produced by a strong white light from an interior sphere passing through a concentric layer of vapors of many substances, each of which absorbs those particular rays that correspond to their own periods of vibration. The light, thus deprived of many definite rays, indicated their absence when its spectrum was cast upon a screen by the appearance of dark lines—the images of the slit through which the light passed—corresponding always to the wave-lengths absorbed. It must not be assumed that these lines of absorption are regions of total darkness. The particles set in vibration by the rays absorbed will naturally give out some light of this same vibration period, but the light emitted is so small in comparison with the rays from the original source which pass through unmolested that the image cast upon the screen will give the appearance of almost total darkness.
Now when a substance is yellow in color we can readily ascertain that the spectrum of the light it reflects is lacking in a number of rays of various wave-lengths. These rays correspond to the complement of the color reflected, and in the case of a yellow substance belong to that magnitude found in the blue end of the spectrum. If no wave-lengths of the visible spectrum had been absorbed, we should have had the continuous spectrum of white light in the light reflected, i.e., the body itself would not be colored. Colored substances, therefore, absorb the rays of their complementary colors and, consequently, when white light is transmitted through them their spectra will indicate the regions of this absorption by dark bands of varying intensity. The absorption spectrum coincides always with the spectrum obtained from the reflected light.