THE GROWTH OF OUR KNOWLEDGE OF HELIUM. 259 



lines of unknown origin exactly coinciding with those un- 

 known lines which I have already referred to as having 

 been seen in the sun's atmosphere. Some of the un- 

 known lines in that atmosphere, those that we have not 

 been able to see in our laboratories, are identical in position 

 with some of the unknown lines in the nebula of Orion. 



I may remark that as early as 1886 Dr. Copeland had 

 discovered D 3 in the visible spectrum of the nebula, and 

 in a letter to him I had suggested that another line he had 

 recorded at 447 might be Lorenzoni's f\ this he thought 

 to be probable. The matter was set for ever at rest by 

 the photograph which established the presence of 4471 and 

 4026 as well, already noted as a solar line. 



Professor Campbell, of the Lick Observatory, obtained 

 other photographs of the spectrum of the nebula some two 

 or three years after mine was taken. In the following list 

 of lines in my photograph an asterisk denotes that Campbell 

 gives a line nearly in the same position. He recorded 

 no line which did not appear on my photograph. 



3896* 

 3888* 

 401 1 

 4026* 

 4121* 



4143* 



4168 



4390* 



4472* 



4716* 



4924 



5875-8 = D3 

 THE SAME UNKNOWN LINES OCCUR IN THE STARS, 1892. 



About the year 1890 I began the photography of stellar 

 spectra at Kensington, with special reference to their 

 classification on the basis of the chemical constituents 

 established by their spectra. By 1892 several important 

 results had been obtained, while the progress of this branch 

 of science lately has been so considerable that any state- 

 ment regarding the positions of lines, and therefore the 



