REPORT OF THE SECRETARY. 73 



of the wave lengths involved. While our knowledge of the infra-red still remains 

 less complete than that of the visible sijectrnm, both in the numl)er of absorption 

 lines mapped and in the accuracy of determination of their wave lengths, yet the 

 difference in the methods of observation must be recalled. On the one liand are the 

 most powerful gratings with all the advantages of direct photography, while on the 

 other ia only a simple prism, in whose dark spectrum we grope for cold lines and 

 measure their wave lengths indirectly. The results of the latter process are 750 lines 

 determined in wave lengths to an accuracy of 3 parts in 10,000, and besides — what 

 photography does not give — an exact knowledge of the distribution of the sun's 

 energy. 

 Respectfully submitted. 



C. G. Abbot, 



Aid Acting in Charge Aslrophysical Observatory. 

 Mr. S. P. Langley. 



Secretary of the SniUliSonian Inslilulion, Washington, D. C. 



