A HISTORY OF SCIENCE 



the various typical stages of the sidereal evolution; 

 and not merely to define them but to illustrate them 

 practically by citing stars which belong to each of 

 these stages, and to give them yet clearer definition 

 by naming the various elements which the spectro- 

 scope reveals as present in each. 



His studies have shown that the elements do not al- 

 ways give the same spectrum under all conditions; a 

 result quite at variance with the earlier ideas on the 

 subject. Even in the terrestrial laboratory it is possi- 

 ble to subject various metals, including iron, to tem- 

 peratures attained with the electric spark at which 

 the spectrum becomes different from that, for example, 

 which was attained with the lower temperature of the 

 electric arc. Through these studies so-called series- 

 spectra have been attained for various elements, and a 

 comparison of these series-spectra with the spectra of 

 various stars has led to the conclusion that many of 

 the unknown lines previously traced in the spectra 

 of such stars are due to the decomposition products 

 of familiar elements ; all of which, of course, is directly 

 in line of proof of the dissociation hypothesis. 



Another important result of Professor Lockyer's 

 very recent studies has come about through observa- 

 tion of the sun in eclipse. A very interesting point 

 at issue all along has been the question as to what 

 layers of the sun's atmosphere are efficient in produc- 

 ing the so-called reverse lines of the spectrum. It is 

 now shown that the effect is not produced, as formerly 

 supposed, by the layers of the atmosphere lying just 

 above the region which Professor Lockyer long ago 

 named the chromosphere, but by the gases of higher 



78 



