xin.] MULTIPLE SPECTRA. 179 



reversal of the iron lines takes place at a considerable height in 

 the atmosphere of the sun, and he imagined the atmosphere of the 

 sun to be an enormous mass represented by the old drawings of 

 coronas, so that on his view the thickness of the iron vapour which 

 reversed the iron spectrum must have been, at a moderate estimate, 

 10,000 miles, and yet he said that the spectrum of that, and of 

 the light given by the electric spark two millimetres in thick- 

 ness, were absolutely identical ; that is to say, that the fact was 

 that the variation of thickness from two millimetres to 10,000 

 miles made no difference. That was on the one hand ; on the 

 other hand he gave us his theorem, showing that a slight variation 

 of thickness would produce all the changes which Angstrom and 

 others had observed up to that time, and which, it may be 

 added, have been observed since in still greater number. 



We now know that in the case of hydrogen a difference 

 in thickness of a million miles and of a millimetre makes no 

 difference. 



9. The same Substance may have more than one Spectrum,. 



At the time that KirchhofF announced the conclusion that the 

 terrestrial elements as known to chemists existed in the sun, 

 the general idea was that each element had only one spectrum, and 

 that that spectrum was the same whether observed in the sun or 

 in our laboratories. Underlying these general notions was, as I 

 have before stated, the assumption that the " chemical atom," a 

 thing with a definite weight given in all chemical text-books, once 

 got, even the solar temperature was insufficient to simplify it. 



Soon after Kirchhoff had published his papers, three eminent 

 Germans Pliicker and Hittorf 1 and the younger Mitscherlich 

 found that in the case of a great many simple substances, 

 what are called fluted spectra, as well as line spectra, were to 

 be observed. 



1 Phil. Trans. 1865, vol. civ. pp. 1-29, 



N 2 



