v..] SODIUM IN THE SUN. 51. 



ether only, it has to traverse ether plus certain kinds of matter 

 contained in the arc. This association leads to certain vibrations 

 of the ether being used up by the matter, and so the residue only 

 reaches the eye. 



Foucault then continues : 



" To make the experiment in a manner still more decisive, I pro- 

 jected on the arc the reflected image of one of the charcoal points, 

 which, like all solid bodies in ignition, gives no lines ; and under 

 these circumstances the line D appeared to me as in the solar 

 spectrum." 



In 1849, then, we get the statement made that a vapour could 

 absorb the same rays that it radiated, and that the light radiated 

 by sodium vapour occupied the same position in the spectrum as 

 some of the light which was absorbed at the sun. But why ? 

 Was there, then, sodium vapour in the sun ? This conclusion 

 could not be accepted until a valid explanation could be given. 

 If the experiment had been made with cool sodium vapour 

 which might, as we now know, easily have been done the thing 

 might have been at once conceded, but the fact that the same 

 substance could produce a bright line under one condition and a 

 dark one under another was difficult to imagine. 



An explanation, however, was not long forthcoming. One 

 was first given (though not published) by Prof. Stokes, about 

 the year 1852. 



The observational and experimental foundation on which 

 Stokes based his explanation has thus been stated by Sir 

 William Thomson : l 



" 1. The discovery by Fratmhofer of a coincidence between his 

 double dark line, D, of the solar spectrum and a double bright line 

 which he observed in the spectra of ordinary artificial flames. 



"2. A very rigorous experimental test of this coincidence by 

 Prof. W. H. Miller, which showed it to be accurate to an astonishing 

 degree of minuteness. 



] Presidential Address, British Association Meeting, 1871. 



E 2 



