176) oS. P. Langley— Determination of asia) 
containing the D, line, was visible, and its deviation was found 
to be 47° 41’, agreeing with the value viven by the table. It 
was the object of the experiment to find the place of the lower 
invisible band, by groping for it; 7. e., to determine its devia- 
tion by trials with the bolometer at intervals sufficiently close 
to avoid the possibility of missing it altogether. According to 
Briot’s formula, the deviation should be 45° 21’, and in the 
imum effect was obtained nearer 45°15’. The approximate 
position having thus been found, the slit S, was narrowed to 
zm, and the following measurements taken, the horizontal pe 
giving the mean results of a series of thirty exposures of t 
bolometer, as it moved through the spectrum. 
Tasie I.—Determination of the refrangibility of feeble heat-rays. 
Prismatic deviation_........-.-- 45° 027 45° or 45°10" 45°157 45°20 — 
Means of sraivesiomiehe readings.. 4°6 6-0 58 art. 
The maximum reading at 45° 10’ corresponds to a coincl- 
dence of the 2™" bolometer aperture, with the 2™ invisible 
image of the slit, whose position is songht. From a subsiciey 
curve drawn through the points whose codrdinates are 
spectively (c=45° 02’, y=4'6), (w= 45° 07’, y=5'6), (a=45° 10, . 
y=6°0), ete., it was concluded that the deviation of rays whose : 
wave-length is 1"-767 is 45° 10’; and each point in this deter 
mination being obtained hoi the mean of five observations, 
the result is partly free from irregularities caused by chan hee 
in the state of the sky, and minute instrumental variations — 
from extraneous causes, which here become of great relative 
importance, owing to the feeble heat measured. ie 
ubsequent determinations, like the preceding, gave pe the 
deviation of the same ray 45° 06’ and 45° 07’, and from a peep 
sideration of all, the deviation aoe was (instead of AB 21, ve 
iven by Briot’s —. 45° 08’, corresponding to a refracy j 
tive index of 1554 Le 
* These three images, mig composed of rays’of different wave-lengtit’ could 
not all be in the same focus of the same lens ao - same time, since 
tor and objective of this be ase 26 vated were simple lenses. The ‘coat ae 
adjusted by means ofa table of focal dista old peoricey prepared, sO 
_throw a sharp (invisible) i diak of the band to be detected. 
collima- 
so 
