iv.] FORBES'S OBSERVATIONS. 45 



In describing the result of his observation in the Philosophical 

 Transactions, 1 mindful of Brewster's work, he first shows that 

 the deficient rays cannot be due to any absorptive action of the 

 glass prisms employed, since whatever material is employed and 

 whatever the length of the path of the light through the prisms 

 the lines remain constant. He then refers to Brewster's discovery 

 of the lines due to the absorption of the earth's atmosphere, but 

 points out that such lines are neither numerous nor important, 

 as compared with the great mass of solar lines, and that if the 

 lines were all or chiefly due either to the absorptive action 

 of the earth's atmosphere or of any matter which may exist 

 in the planetary spaces, we should have the same lines exhibited 

 in the spectra of the fixed stars. 



Reference is also made to Brewster's idea, suggested by his 

 observations of the absorption spectrum of nitrous acid, that the 

 solar light is originally complete and that the deficient rays 

 have been stopped absorbed in passing through the sun's own 

 atmosphere, which might be supposed to contain nitrous acid 

 or some similar gas as a constituent. 



On this point the following note is appended to Forbes's 

 paper : 



" I do not know with whom the idea of the absorptive action of 

 the sun's atmosphere originated. The editors of the London and 

 Edinburgh Philosophical Magazine (December, 1836) have, however, 

 referred me to the mention of it in Sir John Herschel's writings, 

 particularly his Elementary Treatise on Astronomy, from which I 

 extract the following remarkable passage : ' The prismatic analysis 

 of the solar beam exhibits in the spectrum a series of fixed lines 

 totally unlike those of any known terrestrial flame. This may here- 

 after lead us to a clearer insight into its origin. But before we can 

 draw any conclusions from such an indication we must recollect 

 that previous to reaching us it has undergone the whole absorptive 

 action of our atmosphere, as well as of the sun's. Of the latter 

 we know nothing and may conjecture everything. . . . Tt deserves 



1 Phil. Trans. 1836. p. 450 et seq. 



