KELATIVE WAVE-LENGTH OF LINES OF SOLAS SPECTEUM 513 



readings together so as to include any distance, even the whole spec- 

 trum. The wave-length is calculated for a fixed micrometer constant 

 and then corrected so as to coincide everywhere very nearly with the 

 standards. I suppose the probable error of the relative determinations 

 with the weight 1 in my table to be not far from 1 in 500,000. Ang- 

 strom thinks his standard lines have an accuracy of about 1 in 50,000 

 and ordinary lines much less. 



As to the absolute measure, it is now well determined that Angstrom's 

 figures are too small by about 1 part in 6000. This rests: 1st, on the 

 determination of Peirce made for the U. S. Coast Survey with Ruther- 

 furd's gratings and not yet completely published; 2d, on an error made 

 by Tresca in the length of the standard metre used by Angstrom 1 which 

 increases his value by about 1 in 7700; 3d, on a result obtained in my 

 laboratory with two of my gratings by Mr. Bell, which is published with 

 this paper. Mr. C. S. Peirce has kindly placed his grating at our dis- 

 posal and we have detected an error of ruling which affects his result 

 and makes it nearly coincide with our own. The wave-length of the 

 mean of the two E lines is 



Angstrom (atlas) 5269-12 -5 



Angstrom (Corrected by Thalen) 5269-80 l 



Peirce 5270-16 



Peirce (Corrected by Rowland and Bell) 5270-00 * 



Bell 5270-04 



These results are for air at ordinary pressures and temperatures. The 

 last is reduced to 20 C. and 760 mm. pressure. To reduce to a vacuum 

 multiply by the following : 



Fraunhofer line A C E G H 



Correction factor. .1-000291 1-000292 1-000294 1-000297 1-000298 



o 



The relation between my wave-lengths and those of Angstrom are 



O 



given by the following, Angstrom's value being from p. 31 of his 



memoir: 



A (edge) B (edge) C 



Angstrom 7597-5 6867-10 6717-16 6562-10 6264-31 



Rowland . . 7593-97 6867-38 6717-83 6562-96 6265-27 



Difference 3-5 -28 -67 -86 -96 



1 Thal6n, Sur Spectre du Fer, Societe Royale des Sciences d'Upsal, September, 

 1884, p. 25. 2 From one grating only. 



