156 Britain's Heritage of Science 



have a spectrum identical with that of the sun, but that 

 some of the stars, like Sirius, show only a few very strong 

 lines. Sir David Brewster in 1834 compared the solar 

 spectrum observed by him with Fraunhofer's drawings, and 

 noticing additional lines which change with the position 

 of the sun, ascribed them correctly to effects produced in 

 our own atmosphere. He had already in 1832 referred with 

 approval to Herschel's suggestion that the dark Fraunhofer 

 lines were produced by absorption in the atmosphere of the 

 celestial bodies. An interesting observation which ought 

 to have attracted attention at the time, but, like many 

 others, was only saved from oblivion when the method of 

 spectrum analysis had been permanently established, was 

 made in France by Foucault. In the spectrum of the voltaic 

 arc, he noticed the presence of what we now know to be the 

 sodium lines, and identified them with Fraunhofer's line D. 

 He found further that on passing the sunlight through the 

 arc, these lines became darker, and further discovered that 

 the lines under certain conditions may be reversed hi the 

 arc itself. 



In all these observations many important facts were 

 recorded, but the ideas on radiation were vague at the time 

 and no effort was made to connect it with absorption. Stokes; 

 in his own mind, seems to have been clear on the matter, and 

 in private conversation with Lord Kelvin " explained the 

 connexion of the dark and bright line (of sodium) by the 

 analogy of a set of piano strings tuned to the same note, 

 which if struck would give out that note, and also would be 

 ready to sound it, to take it up, in fact, if it were sounded 

 in air. This would imply absorption of the aerial vibrations, 

 as otherwise there would be creation of energy." 1 At this 

 stage historically, but in ignorance of much of what has 

 been described, Balfour Stewart undertook a comprehensive 

 investigation of the subject of radiation and absorption. 

 Adopting Preevost's views that equilibrium of temperature 

 means a balance between absorption and radiation, he 



1 The quotation is from a letter addressed by Stokes to Sir J. 

 Lubbock (afterwards Lord Avebury) ; see G. G. Stokes, " Memoir and 

 Correspondence," by Sir J. Larmor, Vol. II., p. 75. 



