50 THE CHEMISTRY OF THE SUN. [r:HAP. 



The bright double yellow line observed by Fraunhofer and 

 Brewster is the bright line of sodium vapour referred to in the 

 previous chapter. 



In 1849 this work was continued by Foucault, 1 who used a 

 new method of work by getting a new light-source the voltaic 

 arc produced by the passage of an electric current between 

 charcoal poles charged with salts of different substances. With 

 this new engine he also investigated the double yellow line. 



" The spectrum is marked, as is known, in its whole extent by a 

 multitude of irregularly grouped luminous lines ; but among these 

 may be remarked a double line situated at the boundary of the 

 yellow and orange. As this double line recalled, by its form and 

 situation, the line D of the solar spectrum, / wished to try if it 

 corresponded to it, and in default of instruments for measuring the 

 angles I had recourse to a particular process. 



" I caused an image of the sun, formed by a converging lens, to 

 fall on the arc itself, which allowed me to observe at the same time 

 the electric and the solar spectrum superposed ; I convinced myself 

 in this way that the double bright line of the arc coincides exactly 

 with the double dark line of the solar spectrum. 



" This process of investigation furnished me matter for some 

 unexpected observation. It proved to me, in the first instance, 

 the extreme transparency of the arc, which occasions only a faint 

 shadow in the solar light. It showed me that this arc, placed in the 

 path of a beam of so 7 ar light, absorbs the rays D, so that the above- 

 mentioned line D of the solar light is considerably strengthened 

 when the two spectra are exactly superposed. 



" When, on the contrary, they jut out one beyond the other, the 

 line D appears darker than usual in the solar light, and stands out 

 bright in the electric spectrum, which allows one easily to judge of 

 their perfect coincidence. Thus the arc presents us with a medium 

 which emits the rays D on its own account, and which at the same time 

 absorbs them when they come from another quarter." 



Thus absorption is produced when we destroy the simplicity 

 of the usual medium through which the light passes. Instead of 



1 L'lnstitiit, Feb. 7, 1849. Translated by Professor Stokes in Phil. Mag. vol. 

 xix. p. 194. 



