280 Mr. L. Bell on the Absolute 



counteracts the error quite effectually. In grating I. the 

 correction was one part in 300,000, and in grating II. one 

 part in 60,000. Applying these to the wave-lengths we have 

 for grating I. : — 



Wave-length 5896*11 



Correction — -02 



Corrected w.-l 5896*09 



And for grating II., 



Wave-length 5895*95 



Correction + *10 



Corrected w.-l 5896*05 



Combining these, and giving to grating 1. the greater 

 weight on account of its very small error of ruling, we have 

 finally for the wave-length of D ± at 20° C. and 760 millim. 

 pressure, 5896*08 



or in vacuo, 5897*71. 



It is no easy matter to give any well-founded estimate of the 

 probable error of the above result. So far as experimental 

 errors are concerned, the result with either grating should be 

 correct to one part in two hundred and fifty thousand; but 

 the error in the gratings introduces a complication by no 

 means easy to estimate. As nearly as the writer can judge, 

 however, it seems probable that the error of the final result 

 does not exceed one part in two hundred thousand. For 

 comparison, the values deduced from the work of Peirce and 



o 



of Angstrom are subjoined : — 



Micrometer measure by Rowland, from Peirce' s 



preliminary result . ....... 5896*22 



Thalen's correction of Angstrom 5895*89 



Both being in air at ordinary temperature and 760 millim. 



As neither result was corrected for errors in the gratings, 

 the cause of the discrepancy is obvious. 



Two determinations of absolute wave-length have been 

 published since this work was undertaken by the writer. 

 One is a very elaborate one by Miiller and Kempf, who 

 employed four gratings by Wanschaff, and used the method 

 of minimum deviation. Their results were as follows : — 



Grating . . . (2151) (5001) (8001) (80014) 

 Wave-length . 5896*46 5896*14 5895*97 5896*33 

 By a correction founded on the unwarrantable assumption 

 that the mean value was correct, the above results are brought 

 into apparent agreement. Nothing, how r ever, short of a study 



