176 L. Bell—Absolute Wave-length of Light. 
' exactly known. It was afterwards sent to Breteuil and the meter 
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 pre- 
sented to the United Statesin 1856. It was taken to England in 
1878 and finally determined by direct comparison with the Im- 
perial Yard, Bronze Yard No. 6, and Cast Iron Yards B No. 62 
and C No. 63. 
The subdivisions of R, have been determined with very great 
care, and thus 8% and S$, whose lengths relative to R, are accu- 
rately known, may finally be referred to the ultimate standard 
Type I of the International Bureau. 
Only the 5 spaces of S4 and S$ 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 
S? have the following lengths at the standard temperature. 
Standard Sj =199-99915™™ 
dm S¢= 99-99995 
, Si = 9999923 
5m St = 50-00010 
5m St — 49-99985 
5m St — 49-99901 
5™ S*— 50:00022- 
Similarly the following values were derived for S$. 
Standard S$ =199-99968™™ 
_Sh= 99°99967 
5 S$ = 50-00020 
5m S$= 49:99981 
_S5= 4999931 
4 
As to the degree of accuracy attained in determining S{ and, 
«Prof. Rogers says that including all sources of uncertainty 
either standard may have an error of -0°3y, but the mean of 
the two, since the determinations were independent ought to 
be even more reliable. Taking all things into consideration it 
seems very improbable that the mean value of S$ and S$ can be 
in error by as much as one part in half a million. 
So much for the standards of length. The comparator used 
in the measurements was a very efficient instrument, particu- 
larly suited for the purpose. It consisted essentially of a long 
carriage (4 meter) running on V-shaped ways' and carrying the 
microscope. This carriage slides against adjustable stops, and 
