189 



Kirchoff and Bunsen say, in arguing upon these lines and 

 the hypothesis of their representing the solar dark lines, 

 " It was proved from theoretical considerations that the 

 spectrum of an incandescent gas becomes reversed (that is, 

 that the bright ones become changed into dark ones) when 

 a source of light of sufficient intensity giving a continuous 

 spectrum, is placed behind the luminous gas. From this 

 we may conclude that the solar spectrum with its dark lines is 

 nothing else than the reverse of the spectrum, which the sun's 

 atmosphere would alone produce. Hence, in order to effect the 

 chemical analysis of the solar atmosphere, all that we re- 

 quire is to discover those substances which when brought 

 into the flame produce bright lines coinciding with the 

 dark ones of the solar spectrum." The next step in the pro- 

 cess of the investigation instructs us in the fact that the 

 vapors producing those colored flames are opaque to their 

 own rays. That is to say, if we produce a yellow soda 

 flame, and from it obtain a spectrum, showing the peculiar 

 soda-lines in their bright yellow color* and then impregnate 

 the air with some soda vapor, by volatilizing soda between 

 the flame and the spectrum the bright yellow line becomes at 

 once a black line. This holds true for all the substances 

 which have yet been examined. The colored bright lines 

 are converted into dark lines, if the rays from the colored 

 flames are made to permeate vapors of the same constitu- 

 tion as those which produced the particular spectrum under 

 examination. 



Prof. Kirchoff wishing to test the accuracy of the fre- 

 quently asserted coincidence of the bright metallic and 

 dark solar lines, made the following very remarkable ex- 

 periment which is interesting as giving the key to the solu- 

 tion of the problem regarding the existence of sodium and 

 other metals in the sun. He states, " I obtained a tolerably 



* This beautiful bright yellow line is observable when less than one twenty 

 millionth of soda smoke is mixed with air. 

 [Trans, iv] 26 



