SKETCH OF PROFESSOR STOKES. 745 



ation at Edinburgh in 1871, tliat 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 constitu- 

 ents 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 coin- 

 cidence 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 particu- 

 lar 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 coun- 

 tryman, Mr. Balfour Stewart. The inference which Kirchhoff drew from Pre- 

 vost'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 ab- 

 sorbing 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 ex- 

 actly 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 be- 

 lief 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 Kirchhofl:'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." 



