172 L. Bell—Absolute Wave-length of Light. 
overlapping blue rays which might otherwise have proved 
troublesome. A heliostat enabled the sun’s image to be kept 
centrally upon the slit. 
The method of observation wasas follows: When instrument 
and grating were in exact adjustment, readings were taken on D, 
in the spectra on either side of the slit, and the angle measured ~ 
from three to six times in rapid succession, the last reading 
being of course on the same side as the first. 
Then the grating was rotated about ten degrees, readjusted, 
~ and the process repeated. ) 
The angles observed in one series were combined to eliminate 
errors of setting, while the use of all portions of the circle served 
to correct errors of subdivision, since the number of indepen- 
dent series of observations was quite large. 
To eliminate any errors which might be due to imperfections 
of figure in the gratings they were used in all the four possible 
positions. No such error, however, became apparent either 
from critical examination of the gratings themselves, or from 
the results obtained in the different positions. 
Observations with grating I were begun late in October, 1885, 
and occupied the clear days for a month. Forty-eight series of 
measurements were made, and the agreement between them was 
very satisfactory. After correcting for temperature, thirty-six 
of the number fell within a range of three seconds and the rest 
were clustered closely about them. Observations on the various 
days were as follows: 
Number of 
Date. Series. Angle. 
Oct. 19 il ANS) ILE Uap 
20 1 ASr wana Bud 
22 2 Abe 1487-9 
23 1 45° 1’ 49”-8 
26 4 Ao eA" -3 
On 3 A Seu re D 
31 1 45° 1’ 50”1 
Nov. 3 1 45° 1’ 48”.6 
4 3 Ace Arias: 
5 9} AeA eo 
10 4 Abe nil Aves 
11 6 ABS AA QTET, 
16 8 AM VY SY o) 
7 5 ABs) Al Avian, 
20 6 LUGS A Aus 
All the above were in the third spectrum to which measure- 
ments were in the main confined as in it the definition was 
particularly good, and it being the highest order which could be 
conveniently observed, an error in the angle would produce the 
