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



It should be noted that the extreme variation in the above 

 series is 2 '2, very nearly 0"'5, or one part in two hundred 

 thousand. 



In the case of IY the coefficient appeared to be somewhat 

 smaller than that of S%. The range of temperature secured 

 was not large but as nearly as could be ascertained the coeffi- 

 cient is about \W"\ per meter per degree, while that of the 

 standards is IT^'9 per meter per degree. However, since the 

 measurements of <p made with IV were distributed with a tol- 

 erable degree of symmetry on both sides of 20°, any error due 

 to an inexact value of the coefficient of expansion would appear 

 mainly in the probable error in <p The variation found would, 

 as a matter of fact have changed the final value of cp by less 

 than 0"-2. 



The comparisons of IY made near 20° were as follows : 



G = D mi S a 2 + 35 6 -8 



G = " + 35-5 



G = " + 35-6 



G = " + 36-0 



G = " + 34-0 



G = " + 35-8 



G = * " + 34-6 



G = " + 36-3 



G = " + 33-3 



G= " + 36-7 



Combining these and the other observations, 



39465 spaces = G = Dn^S*, + 9^1 at 20°. 



The probable error of the relations found for III and IV 

 can hardly exceed one part in a million so far as the distance 

 between the terminal lines selected is concerned. These termi- 

 nal lines were varied at each comparison so that while each of 

 the above relations represents 39,465 spaces, the lines meas- 

 ured between, though in the same vicinity, are seldom or never 

 identical. 



In gratings I, II, III the number of spaces was very easily 

 counted as the dividing engine automatically rules every hun- 

 dredth line longer, and every fiftieth line shorter, than the 

 others, hi grating IY the number of spaces was found readily 

 enough by ruling at a known temperature the terminal lines of 

 a test plate almost exactly a decimeter long, and containing a 

 known number of lines. A comparison of this with the 

 grating gave the quantity required. 



