228 



Sir Norman Lockyer. 



[Jan. 30, 



which I had first observed and named in 1869, we then knew 

 nothing either on the earth or in the stars ; the solar line D 3 being its 

 only representative. The question of the relative temperatures of 

 stars became of great importance in relation to the questions thus 

 raised, but it was not till 1892 that I was able to approach it by means 

 of photography. The interval was spent chiefly in researches bearing 

 upon solar and terrestrial changes in spectra when differences of thermal 

 and electric energies were obvious. 



In a paper on stellar spectra in relation to temperature, communicated 

 to the Royal Society in 1902,* I gave an account of an attempt at a 

 temperature classification of stars, utilising the fact that an extension 

 of spectra into the ultra-violet is produced by increased temperature, 

 and further that a lower temperature in an atmosphere above a photo- 

 sphere would increase the absorption in the blue end. The classification 

 arrived at was based on photographs obtained with instruments having 

 prisms and lenses made of glass which has a strong absorbing effect on 

 the ultra-violet rays. 



The general results of the discussion was the conclusion that the 

 stars so far considered might be divided into two series, one of 

 ■ascending, the other of descending temperature. Further, that the 

 classification proposed was justified both by the relative extensions 

 of the spectra into the ultra-violet and by the temperature sequence of 

 the few typical lines then available for study. 



By 1899, laboratory work on the spectra of different substances 

 under different conditions, and the discovery of a terrestrial source of 

 helium by Eamsay, which enabled me to investigate the complete 

 spectrum, had so far facilitated the study of the typical lines in the 

 various stellar spectra, that I felt myself justified in attempting to 

 classify the stars in relation to the chemical sequence revealed by the 

 presence of gaseous and metallic lines, using especially the lines of 

 helium and hydrogen and the " enhanced " and arc lines of the metals. 

 In this way I hoped to be able to test the classification of 1892 based on 

 the relative lengths of spectra. 



An account of this research was published in the Proceedings of the 

 Eoyal Society (vol. 65, pp. 186—191, 1899), and ultimately the 

 complete results obtained were included in a " Catalogue of 470 

 Brighter Stars Classified According to their Chemistry."! 



In this catalogue the stars were arranged in sixteen groups along a 

 temperature curve with its apex in the central portion. On the 

 assumption that the chemical changes were due to temperature, 

 including in that term the possible results of electrical energy, the 

 general arrangement of the stellar groups in the order both of 

 ascending and descending temperatures was indicated, the group 

 * ' Phil. Trans.,' A, vol. 184, p. 688. 



t ' Publications of the Committee on Solar Physics,' London, 1902. 



