Masterpieces of Science 



a certain double dark line which had already been 

 found to correspond exactly in position with the 

 double bright line forming the spectrum of the 

 glowing vapour of sodium, was intensified when 

 the light of the sun was allowed to pass through 

 that vapour. This at once suggested the idea 

 that the presence of this dark line (or, rather, 

 pair of dark lines) in the spectrum of the sun is 

 due to the existence of the vapour of sodium in 

 the solar atmosphere, and that this vapour has 

 the power of absorbing the same order of light- 

 waves as it emits. It would of course follow 

 from this that the other dark lines in the solar 

 spectrum are due to the presence of other absorb- 

 ent vapours in its atmosphere, and that the 

 identity of these would admit of being estab- 

 lished in the same way, supposing this general 

 law to hold that a vapour emits the same light- 

 waves that it is capable of absorbing. 



Kirchhoff was soon able to confirm his views by 

 a variety of experiments. The general prin- 

 ciples to which his researches led in other 

 words, the principles which form the basis of 

 spectrum analysis are as follows: 



1. An incandescent solid or liquid gives a con- 

 tinuous spectrum. 



2. A glowing vapour gives a spectrum of bright 

 lines, each vapour having its own set of lines, so 

 that from the appearance of a bright-line spec- 

 trum one can tell the nature of the vapour or 

 vapours whose light forms the spectrum. 



3. An incandescent solid or liquid shining 



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