Wave-length of Light. 371 



Mascarfc 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 



Kurlbaum 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 commen- 

 surate in accuracy with the relative wave-lengths as measured 

 by Prof. Rowland. This hope has proved in a measure illu- 

 sory, 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 re- 

 membered that any and every method involves the uncer- 

 tainty of the standards of length, an uncertainty not to be 

 removed until a normal standard is finally adopted and exact 

 copies of it distributed. And as far as experimental diffi- 

 culties are concerned, the next order of approximation will 

 involve a large number of small but troublesome corrections, 

 such as the effect of aqueous vapour on atmospheric refrac- 

 tion, varying barometric height, the minute variations in the 

 grating-space, failure of thermometer to give temperature of 

 grating exactly, and countless others which will suggest 

 themselves only too readily. 



Aside from the use of gratings, decidedly the most hopeful 

 method as yet suggested is that due to Michelson and Morley*. 

 Theoretically the plan is particularly simple and beautiful, 

 consisting merely in counting off a definite number of inter- 

 ference-fringes by moving one of the interfering-mirrors and 

 measuring, or laying off upon a bar, the resulting distance. 

 The mechanical difficulties in the way are, however, formid- 

 able, and whether or no they can be surmounted only per- 

 sistent trial can show. The possible sources of error are of 

 much the same type and magnitude as those involved in the 

 comparison of standards of length; and if these errors are 

 avoided, the uncertainty concerning the standards still remains. 

 Whether or no the practical errors of the method are greater 



* This Journal, Dec. 1887, p. 463. 



