THE 



LONDON, EDINBURGH, and DUBLIN 



PHILOSOPHICAL MAGAZINE 



AND 



JOURNAL OF SCIENCE. 



[FIFTH SERIES.] 



APRIL 1888. 



XXX. The Absolute Wave-length of Light. By Louis Bell, 

 Fellow in Physics in Johns Hopkins University*. 



THIS paper contains the final results of the research 

 partially reported in this Journal for March 1887. 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 sub- 

 ject, and to discuss critically certain portions of the investi- 

 gation 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 

 arise in connexion with the work of recent experimenters. With 

 this preliminary notice is presented the first half of the paper. 



Hist 



Fraunhofer's first paper on the lines which bear his name 

 marks a new era in the science of optics. Up to that point 

 any careful study of spectra had been impossible for lack of 



* Communicated by the Author. 

 Phil. Mag. S. 5. Vol. 25. No. 155. April 1888. S 



