Wave-length of Light. 27 5 



the metre was determined with great exactness by elaborate 

 comparisons with Type I. of the International Bureau of 

 Weights and Measures. 



III. "Bronze 11," a primary copy of the Imperial Yard, 

 presented to the United States in 1856. It was taken to 

 England in 1878; and finally determined by direct compari- 

 son with the Imperial Yard, Bronze Yard No. 6, and Cast 

 Iron Yards B No. 62 and C No. 63. 



The subdivisions of R 2 have been determined with very 

 great care; and thus Si and S 2? whose lengths relative to K 2 

 are accurately known, may finally be referred to the ultimate 

 standard Type I. of the International Bureau. 



Only the 5-centim. spaces of Si and S 2 were investigated 

 by Prof. Rogers; but these were determined by various methods 

 under widely different conditions, and their relations to the 

 standards with which they were compared may be regarded 

 as definitely known. From a combination of all results the 

 subdivisions of Si have the following lengths at the standard 

 temperature : — 



millim. 



Standard Si = 199*99918 



din.! S?= 99*99995 



dm. 2 S?= 99-99923 



5-cm. t S?= 50-00010 



5-cm. 2 S?= 49-99985 



5-cm. 3 S?= 49-99901 



5-cm. 4 S?= 50-00022 



Similarly the following values were derived for S 2 : — 



millim. 

 Standard S 2 = 199-99968 

 dm.! S 2 = 100-00001 

 dm. 2 S 2 l = 99-99967 

 5-cm.i S^= 50-00020 

 5-cm. 2 SS= 49-99981 

 5-cm. 3 S^= 49-99931 

 5-cm. 4 S 2 = 50-00042 

 As to the degree of accuracy attained in determining SJ 

 and S 2 , Prof. Rogers says that, including all sources of un- 

 certainty, either standard may have an error of ±0*3^; but 

 the mean of the two, since the determinations were inde- 

 pendent, ought to be even more reliable. Taking all things 

 into consideration, it seems very improbable that the mean 



U2 



