266 Mr. L. Bell on the Absolute 



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 Angstrom 

 are now known to be too small by as much as one part in 

 seven or eight thousand, as has been shown by Thalen, in 

 his monograph Sur le Spectre du Fer ; and since Angstrom'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 absolute wave-length as nearly as possible com- 

 mensurate in accuracy with the micrometrical observations, 

 that the experiments detailed in the present paper were under- 

 taken. Only the work with glass gratings has been as yet 

 completed ; but since the relative wave-lengths, which are 

 intrinsically of far greater importance, are now ready for 

 publication, and have been reduced by the value herein given, 

 the result is published, leaving for further work with speculum 

 metal gratings its final confirmation or correction. 



This portion of the determination is delayed awaiting better 

 facilities for carrying it out, but the writer intends under- 

 taking it at the earliest possible moment, and hence leaves for 

 a future paper the complete discussion of the problem. 



The writer desires here to express his deep obligations to 

 Prof. Rowland, under whose guidance the work has been 

 carried on, and to whom a very important correction is due ; 

 and to Profs. W. A. Rogers and C. S. Peirce for information 

 given and courtesies extended. 



Experimental. 

 The determination of absolute wave-length involves two 

 quite distinct problems — first, the exact measurement of the 

 angle of deviation of the ray investigated, and second, the 

 measurement of the absolute length of the gratings used. 

 Each portion of the work involves its own set of corrections, 

 frequently quite complicated and difficult, but it is the latter 

 part that is peculiarly liable to errors, which will be treated 

 in detail further on. As to the former part, several import- 

 ant questions arise at the very outset. First is the choice 

 between transmission- and reflection-gratings. The principal 



