Stokes learning, in 1850, that experiments had been made by professor 2Iiller of Cain bridge, to test with great accuracy Fraunhofer's assertion as to the exact er incidence of the double bright line of a salted flame with the double dark line of the solar spectrum, gave for the first time the physical explanation of the phenomenon. He compared the salt-flame to a space full of tuning-forks or piano-forte wires all tuned to the same note. When they are in vibration they, of course, give out this note—similarly the salt-flame the bright lines. When, however, sounds are produced in their neighborhood, as they naturally vibrate to one definite note, they will be set in vibration by it (i.e., will absorb it) if it be part of the sound.—Thus sound has passed through such a space has had this note eliminated from it—similarly the salt flame seizes these yellow rays from white light passing through it. This ingenious and satisfactory explanation shows at once that the line D proves the existence of salt (or sodium) in the atmosphere of the sun. Stokes's theory was not published. except in so far as it was annually given by sir IV. Thompson (q.v.) in his lectures in Glascow—so that it was independently discov ered, or all but discovered, by various other philosophers some 8 or 10 years later. The earliest of these was Balfour Stewart of the Kew observatory, who proved by rensoning and experiment that a body's absorbing power for any ray of light or heat is equal to its radiating power for the same, Angstrhm all but made the rediscovery. Finally Kirch
hoff, by reasoning similar to that of Stewart, and by actually reversing the spectra of certain substances, arrived at the same results; and, in conjunction with Bunsen, applied them to chemical analysis, with the immediate result of discovering two new metals.
One of the most valuable parts of Kirchhoff's investigation is his map of the solar spectrum with its dark lines: side by side with which is a spectrum containing the bright lines given by various metals volatilized in an electric spark. The sunlight is admitted through the upper half of the slit, the light from the burning metal through the lower— and trios the two are subject to precisely the same deflections by the train of prisms.
The applications of the spectrum analysis are becoming more numerous every day. Huggins has lately shown that the spectra of planetary iiebulee, and of the tails of, comets, consist of a few bright lines only—indicating that these bodies—or, at :61 events, those portions of them from which their light proceeds—are masses of incandescent vapors or gases.
Again, Stokes has traced, by the alteration of the absorption bands produced by the coloring matter of blood, the oxidation and reduction winch constantly take place in this substance, and its connection with the distinction between venous and arterial blood.
•