THE 



AMERICAN JOURNAL OF SCIENCE 



[THIRD SERIES.] 



Akt. XXII. — The Absolute Wave-length of Light • by Louis 



Bell. 



This paper contains the final results of the research 

 partially reported in this journal for March 1886. In view of 

 the wide discrepancies in the value of this physical constant 

 as determined by various observers and methods, it has seemed 

 desirable to give in brief the history of the subject, and 

 critically to discuss certain portions of the. investigation which 

 have proved stumbling-blocks in the past. I refer particularly 

 to the verification of the standards of length employed, and to 

 those errors of ruling in the gratings which may, and usually 

 do, produce errors in the result obtained. 



The first portion, of this paper will be devoted to the 

 methods and results of the pioneers in this work and the 

 methods, apparatus, and standards of length employed in the 

 present investigation. 



The second portion will contain the details of the experi- 

 mental work, together with a discussion of the final results and 

 those questions of theoretical and practical interest which raise 

 themselves in connection with the work of recent experi- 

 menters. With this preliminary notice is presented the first 

 half of the paper. 



Historical. — Fraunhofer's first paper on the lines which 

 bear his name marks a new era in the science of optics. Up 



Am. Jour. Sci — Third Series.— Vol. XXXV, No. 208.— April, 1888. 

 16 



