366 L. Bell — Absolute Wave-length of Light. 



The agreement of relative wave-lengths as determined by 

 different experimenters unfortunately gives no measure as to 

 the accuracy of the work. The relative wave-lengths as de- 

 termined by Miiller and Kempf and by Kurlbauin agree in gen- 

 eral to within 1 part in 100,000 : the absolute wave-lengths 

 assigned by these experimenters vary by more than 1 part in 

 30,000. 



A very ingenious flank movement on the problem of abso- 

 lute wave-length has been made by Mace de Lepinay. His 

 plan was to use interference fringes in getting the dimensions 

 of a block of quartz in terms of the wave-length, and then to 

 avoid the difficulties of the linear measurement by obtaining 

 the volume through a specific gravity determination. His re- 

 sults do not indicate, however, experimental accuracy as great 

 as can be obtained by the usual method, and the final reduc- 

 tion unfortunately involves a quantity even more uncertain 

 than the average standard of length, i. e., the ratio between the 

 meter (?) and the liter. 



It may be interesting here to collect the various values 

 which have been given for the absolute wave-length within 

 recent years. Results are for the line D r 



Mascart 5894-3 



Van der Willigen 5898*6 



Angstrom 5895-13 



Ditscheiner 5897*4 



Peirce 5896*27 



Angstrom corrected by Thalen 5895*89 



Miiller and Kempf 5896*25 ' 



Mace de Lepinay 5896*04 



Kurlbauin 5895*90 



Bell 5896*18 



These figures are discordant enough. When beginning the 

 present work, I had hoped that it would prove possible, to make 

 a determination of absolute wave-length commensurate in 

 accuracy with the relative wave-lengths as measured by Prof. 

 Rowland. This hope has proved in a measure illusory, by 

 reason of the small residual errors of the gratings and the 

 greater uncertainty involving the standards of length. I feel 

 convinced, however, that the result reached is quite near the 

 limit of accuracy of the method. It should be remembered 

 that any and every method involves the uncertainty of the 

 standards of length, an uncertainty not to be removed until 

 a normal standard is finally adopted and exact copies of it dis- 

 tributed. And as far as experimental difficulties are con- 

 cerned, the next order of approximation will involve a large 

 number of small but troublesome corrections, such as the effect 



