178 LI. Bell— Absolute Wave-length of Light. 
plate, the required angle could be deduced with great exactness. 
It differed so little from 90° however, that the correction pro- 
duced, barely one part in a million, was entirely negligible. 
After all reductions and corrections, the following series of 
values were obtained for the grating spaces of gratings I and 
i: 
Series. Grating I. Standard. 
1 0°00250023™™ SZ 
2 0:00250016 G 
3 0°002500138 6 
4 0°00250015 se 
5 0°00250018 & 
6 0°00250021 S4 
7 0°00250023 ce 
. 8 0:00250023 “c 
9 0:00250023 cc 
Mean value adopted after weighting and combining the above 
observations was: 
0°002500194""™+10 
The probable error thus appears to be not far from one part in 
two hundred and fifty thousand. The difference in the results 
obtained from the two standards seems to be purely accidental 
as appears from the measurements on grating II. 
Series. Grating IT. Standard. 
Dy 00085 1888"" S4 
2 0°003851883 $5 
3 0°00351885 sf 
4 0:00351886 ‘ 
5 0°00851883 se 
6 0:00351893 Sa 
7 0°003851888 ts 
8 0:00351888 My 
9 0°00351888 : 
Mean adopted, 0:003518870--10 
The probable error appears to be rather less than in the 
measurements of grating I. As however the angular deter- 
minations made with I are the better, so far as probable errors 
of observation are concerned the result from the two gratings 
are about equal in value. 
Computing now the wave-length corresponding to the given 
values of g and d for each grating, we have finally for the wave- 
length of D, at 20° C. and 760™™ pressure : 
From Grating I uncorrected, 5896°11 tenth meters. 
From Grating II is 5895°95 rf 
