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



I now found for the same quantities : 



Dm 2 S a 1 = Dri^S*, + 0-64//, direct, 

 DmjS^ = Dm 1 S a I + 0-60/*, from R B 

 Dm : S\ = Dm S a n + 1-60/./, direct 

 Dm 1 S a 2 = Dm 2 S a 2 + 1-65//, from R B 



All these relations being for 16°'6Y C. 



The standard S% was taken to Berlin during the summer 

 and through the kindness of Dr. Nieberding, Director of the 

 Normal Aichungs Commission, I was enabled to have it com- 

 pared with R, 8 the standard meter to which the wave length 

 measurements of Muller and Kempf, and Kurlbaum had been 

 referred. From this comparison was derived the relation : 



S\ - 1-68// (±0-15/0 =i\ 



On returning to Baltimore the first step was to redetermine 

 the length of S%. A series of comparisons was therefore insti- 

 tuted between it and the steel yard and meter A 4 , the relation 

 of which to R 2 was accurately known, A 4 having been traced 

 and determined by Prof. Rogers and furnished by him to the 

 Johns Hopkins University. Only half of this standard is sub- 

 divided to decimeters but a series of comparisons with the 

 various pairs of decimeters gave the relation, 



S% + V3/x = £A 



This result taken together with the relations found between 

 S a j and S a „ made it tolerably clear that a change had taken 

 place in the speculum metal standards, and to obtain a further 

 confirmation Prof. Rogers kindly consented to give them a 

 rigid examination and again compare them with all attainable 

 accuracy to R„. His results for S a 2 were as follows : 



DnijS 1 , = Dm a S\ + 1-70/* 

 S\ + 1--0A* = *A 



There is no escape from the conclusion, therefore, that the 

 speculum metal bars S t and S„ have changed both in absolute 

 length and the relative lengths of their parts. Here are two 

 bars of the same shape, mass, material, and constant of expan- 

 sion. Each had the relation between its halves determined in 

 the early part of 1885. Two years later these relations are 

 found to have changed by at least l,u, and an independent de- 

 termination by the original observer confirms this result in the 

 most unequivocal way. Further, the original observer recom- 

 pares one of these standards with the standard from which it 

 was originally determined and finds a change of l/i. 



It should be borne in mind that with the comparator used 

 by me in this work, 1/u is completely outside of any possible 

 errors of observation. The microscope used was especially 



