514 HENRY A. ROWLAND 



Da A Peirce's line 



Angstrom 5895-13 5889-12 5708-45 5623-36 5454-84 



Eowland . 5896-08 5890-12 5709-56 5624-70 5455-68 



Difference -95 1-00 1-11 1-34 -84 



E E bi F 



Angstrom 5269-59 5268-67 5183-10 5138-78 4860-74 



Rowland , . 5270-43 5269-65 5183-73 5139-47 4861-43 



Difference -84 -98 -63 -69 -69 



o 



Angstrom 4702-44 4307-25 



Rowland , . 4703-11 4307-96 



Difference -67 -71 * 



The greatest variation in these differences is evidently due to the 

 poor definition of Angstrom's grating by which the numbers refer to 

 groups of lines rather than to single ones. Selecting the best figures, 

 we find that Angstrom's wave-lengths must be multiplied by 1-00016 to 

 agree with Bell, while the correction for Angstrom's error of scale 

 would be 1-000110. 



It is impossible for me to give at present all the data on which my 

 determinations rest, but I have given in Table I many of the coinci- 

 dences as observed with several gratings, the number of single readings 

 being given in the parenthesis over each set. 



Table II gives the wave-lengths as interpolated by the micrometer. 

 It is scarcely possible that any error will be found (except accidental 

 errors) of more than -02, and from the agreement of the observations 

 I scarcely expect to make any changes in the final table of more than 

 01, except in the extremities of the spectrum, where it may amount 

 to -03 in the region of A and H lines. The wave-lengths of weight 

 greater than 1 will probably be found more exact than this. The lines 

 can be identified on my new photograph of the spectrum down to 5790. 

 Below this there is little trouble in finding the right ones. All maps 

 of the spectrum, especially above F, are so imperfect that it is almost 

 impossible to identify my lines upon them. The lines can only be prop- 

 erly identified by a power sufficient to clearly divide & 3 and & 4 . Some of 

 them are double and most of these have been marked, but as the table 

 has been made for my own use, I have not been very careful to examine 

 each line. This will, however, be finally done. Micrometric measures 



