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



A long series of comparisons with the Imperial Yard and its 

 copies in 1878, showed systematically a shortening relative to 

 the Imperial Yard of over 1/y.. Although further measure- 

 ments have tended to somewhat lessen this discrepancy it 

 seems to be sufficient, considering the fact that " No. 11 " and 

 the Imperial Yard are of the same shape, material and mass, 

 and were compared on thje same apparatus as during the orig- 

 inal comparisons in 1857, and at nearly the standard tempera- 

 ture, to establish the fact of a real change. While 3 or 4// is 

 absolutely a small quantity, its systematic appearance under 

 conditions almost identical with those of the original measure- 

 ment can hardly be ascribed to experimental errors. The other 

 cases cited in the above mentioned paper tend to confirm this 

 conclusion. 



The gradual and sometimes very irregular changes that are 

 known to take place in both the bulbs and stems of thermom- 

 eters, would lead one to expect that glass standards of length 

 would be liable to similar changes, though probably in far less 

 amount. It was, therefore, with special interest that I examined 

 glass Decimeters III and IY belonging to the Coast Survey, and 

 used by Peirce in his wave-length measurements. These scales 

 are on plate glass, of the same dimensions, and having coeffi- 

 cients of expansion not widely different. A series of compari- 

 sons made at a nearly constant temperature of 16 0, 5 C. gave 

 the direct relation 



IH=]T+ 2*l/i 

 While the same relation deduced from Peirce's direct com- 

 parison by applying the coefficients of expansion assigned by 

 him, is 



m=iv+ i-3/i 



The defining lines on both standards are fine and sharp, and 

 unless Peirce's coefficients are grossly in error, the evidence of 

 change between 1879 and 1887 is very strong indeed. 



Having now the exact present relation of S a „ to the original 

 standard P 2 , it remained only to investigate the difference be- 

 tween this result and the length of S a 2 as deduced from the 

 Berlin comparisons. I have been unable to obtain the details 

 concerning P 78 , the standard used in these comparisons, but it 

 was determined by comparison with the standard meter of 

 the International Bureau. The comparisons of S a 2 with P 78 

 were carefully made by two observers, and it is probable that 

 the result represents the relation between these standards with 

 considerable exactness. It should, however, 'be borne in mind 

 that the microscopes had power of only 50 diameters, and that 

 the bars in question are of very different material and mass, 

 thus giving a chance for small errors due to varying tempera- 

 ture. 



