172 8. P. Langley—Selective Absorption of Solar Energy. 
train, 5 meters beyond the grating itself. In the gratings em- 
ployed one of the second spectra is very feeble, or almost lacking. 
The rays of the second spectrum are necessarily superposed on 
those of double the wave-length in the first; and as all evidence 
of solar radiation in the most sensitive apparatus at the sea- 
level dies out near 4 = 03 in the ultra violet, it follows that 
we can measure down in the first spectrum as far as 2 = 06, or 
in fact further, without any fear whatever of our results being 
affected by the underlying second spectrum, even if that were a 
strong one. Underlying 0”-7, which is near the limit of the 
t rst spectrum, is 0”:35 in the second. where heat is 
practically still negligable. We have, therefore, knowing the 
amount of heat in the second spectrum at 0:5, and knowing 
that our ultimate point of measurement at 1”-0 in the first 
spectrum overlies 05 in the second, the means of asserting 
with confidence that no considerable error can be introduced 
from this cause, after an allowance has been made here for the 
minute effect of this second spectrum. An allowance is also 
made to reduce the effect to that which would have been ob- 
served with a grating so coarsely ruled as to cause no consider: 
able deviation from the slit of any portion of the spectrum 
measured. The bolometer (in a constant position relative to 
the coneave mirror such that the optical axis of the latter 
bisected the angle between its central thread and the center of 
the grating), was moved, together with the mirror, by @ tan- 
gent screw in arc, so that the spectrum appeared to traverse its 
ace. 
The actual angular deviation of any ray under examination 
was obtained from a divided circle on which the arm carrying 
both mirror and bolometer moved. A particular description 
is not given, as the whole apparatus was replaced by a more 
perfect one later. That actually used will be intelligible by 
the sketch, fig. 1, where S is the slit, G the grating, M the cou 
cave mirror, B the bolometer, and C the divided circle. 
he light came from the silvered mirror of a heliostat, pass: 
The mirror and the bolometer, with their attachments, were 
fastened to this movable circle. 
An allowance has been made for the absorption of speculum 
metal and silver, but the absorption of the iron strips of the 
bolometer has only been indirectly allowed for. This has been 
done by comparison with the action of a bolometer with lamp- 
f, 
