ation at Edinburgh in 1871, that Stokes (at least as early as 1852) had fully apprehended the physical basis of spectrum analysis, and had pointed out how it should be applied to the detection of the constituents of the atmospheres of the suns and stars. Balfour Stewart's experiments and reasoning date from 1858 only, and those of Kirchhoff from 1859."
Prof. Stokes, however, gives due credit to Kirchhoff. Thus, in his Presidential Address to the British Association, in speaking of the applications of the spectroscope, he says:
"But how shall we find in such distant objects as the stars an analogue of the bell which we have assumed in the illustration drawn from sound? What evidence can we ever obtain, even if an examination of their light should present us with rays of definite refrangibility, of the existence in those remote bodies of ponderable matter vibrating in known periods not identical with those corresponding to the refrangibilities of the definite rays which we observe? The answer to this question will involve a reference, which I will endeavor to make as brief as I can, to the splendid researches of Prof. Kirchhoff. The exact coincidence of certain dark lines in the solar spectrum, with bright lines in certain artificial sources of light, had previously been, in one or two instances, observed; but it is to Kirchhoff we owe the inference, from an extension of Prevost's theory of exchanges, that a glowing medium which emits bright light of any particular refrangibility necessarily (at that temperature at least) acts as an absorbing medium, extinguishing light of the same refrangibility. In saying this, it is but just to mention that, in relation to radiant heat (whence the transition to light is easy), Kirchhoff was preceded, though unconsciously, by our own countryman, Mr. Balfour Stewart. The inference which Kirchhoff drew from Prevost's theory thus extended, led him to make a careful comparison of the places of the dark lines of the solar spectrum with those of bright lines produced by the incandescent gas or vapor of known elements; and the coincidences were in many cases so remarkable as to establish almost to a certainty the existence of several of the known elements in the solar atmosphere, producing by their absorbing action the dark lines coinciding with the bright lines observed. Among other elements may be mentioned, in particular, hydrogen, the spectrum of which, when traversed by an electric discharge, shows a bright line or band exactly coinciding with the dark line C, and another with the line F.
"Now, Mr. Huggins found that several of the stars show in their spectra dark lines coinciding in position with and F; and what strengthens the belief that this coincidence, or apparent coincidence, is not merely fortuitous, but is due to a common cause, is, that the two lines are found associated together, both present or both absent. And Kirchhoff's theory suggests that the common cause is the existence of hydrogen in the atmospheres of the sun and certain stars, and its exercise of an absorbing action on the light emitted from beneath."
