L.Bell — Absolute Wave-length of Light. 355 



Hence combining these and reducing them to the standard 

 temperature of 20° we have : 



60000 spaces = 5G = 15 cm S a 2 + 5^-2 at 20° 



The micrometer constant here used was that of the new 



micrometer where Id = 0"-257. 



In precisely the same way Grating II was remeasured, the 



six series giving the following relations 



5G = 15 cm S% + 15^-4^ 

 5G = 15 cra S a „ + 154-9 

 5G = 15 cm S% + 154-5 

 5G = 15 om S\ + 152-4 

 5G = 15 cm S% + 154-9 

 5G= 15 cm S a + 162-4 



► At 19°-8 C. 



Combining and reducing these results as before we have the 

 equation 



42640 spaces = 5G = 15 cm S a 2 + 39^9 at 20° 



The temperature variation in the two days of observation was 

 only 0°-2. 



Gratings III and IV were then measured. In this case a 

 large number of comparisons were obtained at both high and 

 low temperatures with the object of detecting any differences 

 which might exist between the coefficients of expansion of 

 the gratings and those of the speculum metal standards. Ill 

 and IY being a little over a decimeter in length were very 

 easy to measure, particularly since the lines were very sharp and 

 of approximately the same width as those on the standards. 



Ill proved to have sensibly the same coefficient as the stan- 

 dards. I subjoin the comparisons made at or very near 20°. 



G = 



Dn^S*, 



+ 32 d -< 



G = 



a 



+ 33-0 



G = 



c« 



+ 32-7 



G = 



a 



+ 33-2 



G = 



a 



+ 32-3 



G = 



u 



+ 32-6 



G = 



a 



+ 34-5 



G = 



c. 



4- 33-4 



G = 



a 



+ 34-2 



G = 



a 



+ 32-6 



Combining these and other series of observations gives 

 finally 



28418 spaces = G = Dm 1 S a 2 + 8^5 at 20° 



