L. Bell—Absolute Wawe-length of Light. 173 
minimum effect. The spectra on both sides of the slit were 
about equal in brilliancy and definition. 
The observations were weighted as nearly as possible accord- 
ing to the favorable or unfavorable conditions under which 
they were made, and when finally combined gave as the value of 
the angle of deviation for grating I, 
@=45° 1' 48"-24--0"'11. 
The above probable error is equivalent to a little less than 
one part ina million and can introduce no sensible error into 
the resulting wave length. 
Other work intervened and the measurements with grating 
II were not taken up until early in the succeeding March. 
Precisely the same method of observation was employed and 
the results were nearly as consistent and satisfactory. 
The observations by days were as follows: 
Date, 1886. = Number of Series. Angle. 
March 6 2 BAA Bi) Wi eli) 
10 1 ADP A586 
11 i ADTs ay Aca. 
15 1 AD) GDL hay Av-0 
16 6 AD A ESIC) 
re 6 AD AMO Oued 
18 7 AOR AL 5 OF el 
23 6 42° A" 58"°3 
When collected thus by days the observations do not appear 
to agree nearly as well as those made with grating I, particu- 
larly since a solitary wild reading, that of March 15, is retained. 
The distribution of the various readings, however, is such that 
after weighing and combining the final result is by no means 
deficient in accuracy. It is, 
p = 42° 4’ 59"28 + 07-2 | 
The above probable error amounts to about one part in six 
hundred thousand. The observations with grating II were 
uniformly in the fourth order of spectrum. 
Throughout the measurements with both gratings the temper- 
ature was kept within a few degrees. 20° C. had been selected 
as the standard temperature and the variation was rarely more 
than two or three degrees on either side of that figure. The 
question of temperature determination is a serious one in case 
of glass gratings, for it is very hard to tell what heating effect 
the incident beam has on the grating, and equally hard to 
measure that effect. It is hardly safe without extraordinary 
precautions to assume that the grating has the same tempera- 
ture as the air near, and it is such a bad conductor that it 
would not easily assume the temperature of the apparatus. In 
