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THE 
AMERICAN JOURNAL OF SCIENCE. 
[THIRD SERIES.] 
Art. XX.—On the Absolute Wave-length of Light; By Lovis 
BELL, Fellow in Physics in Johns Hopkins University. 
Up to the present time, Angstrém’s map of the solar spectrum 
and with it his determination of absolute wave length, has re- 
mained the final standard of reference in all spectroscopic mat- 
ters. But since Angstrém’s work was published, optical science, 
patticularly that part of it which deals with the manufacture and 
use of diffraction gratings, has made enormous progress. It is 
now possible with the concave grating to measure relative wave- 
lengths with an accuracy far greater than can be claimed for 
any one of the absolute determinations. The numbers given by 
Angstrém are now known to be too small by as much as one 
part in seven or eight thousand, as has been shown by Thalén 
in his monograph “Sur le spectre du Fer,” and since Ang- 
strém’s work but one careful determination has been made. 
This is by Mr. C. S. Peirce and was undertaken some eight 
years since for the U. S. Coast and Geodetic Survey. No full 
report of this work has as yet been published, though it is 
evidently very careful and has already consumed several years. 
Certain results were communicated to Prof. Rowland of this 
University to serve as a standard of reference for his great map 
of the solar spectrum now nearly completed, and it was to serve 
as a check on these results and to furnish a value of the abso- 
lute wave length as nearly as possible commensurate in accu- 
racy with the micrometrical observations, that the experiments 
detailed in the present paper were undertaken. Only the work 
AM. Jour. Sct.—-THIRD SERIES, Vou. XXXII, No. 195.—Marcg, 1887. 
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