186 &. P. Langley—Selective Absorption of Solar Energy. 
progress on the nature of the absorption in the intervals, to de- 
termine whether the newly-observed bands are of solar or ter- 
restrial origin. The part of the spectrum included extends 
from 4 = 0":888 (above H in the violet), to 2 = 2-28 (in the 
newly-observed infra-red region, about two octaves below 
Frauenhofer’s B). 
(As we can, in fact, obtain evidence of heat in ultra-violet 
waves whose length is little more than 0”.8, the length of the 
solar spectrum as now observable by the bolometer is between 
8 and 4 octaves. 
The distant slit is separately exposed at each observation, 
and the extremity of the full swing of the galvanometer needle 
is read. In all these measures the galvanometer is used in the 
same condition of sensitiveness. The slit is opened to a con- 
stant width of 2™™* (except in measuring the very feeble en- 
ergy at the most refrangible end of the spectrum, where the 
width has been increased without prejudice to accuracy, owing 
to the corresponding prismatic expansion of the spectrum itself). 
The same bolometer is used, as a rule, having for this purpose 
i™™ effective aperture (except in measurements at the mos 
refrangible end of the spectrum, where the full aperture of the 
bolometer is used). 
ese observations on the absorption of different air masses, 
for each spectral ray, evidently furnish means for determining 
the curve outside the atmosphere, by the method already indi- 
cated. They also, of course, give us the means of making 4 
map of the whole spectrum, but their use for this latter pul 
pose is incidental. 
2d. The other class of observations is for the special purposé 
of making a spectral map, extending from the line C to the 
lower limit of the Safensted, 
This is carried on by means of the linear bolometer consist- 
ing of a single strip #™™ wide. In this second class of observa- 
tions, a rough map of the whole infra-red spectrum having beet 
prepared, a very limited part of the spectrum (such as that 
included between 15’ of deviation) is gone over several times 
in the course of one day, the measurement being repeated on 
every single minute of are, with a separate opening and closing 
of the slit, and a record made of the full swing of the galvano 
meter needle for each observation. 
_ These observations are entered numerically, and correspond: 
ing charts made on large sheets of section paper. The sam 
narrow region will thus be gone over also on different days, 
and the different charts subjected to a very rigid examination, 
* It will be remembered that, the actual distance of the slit being 5 meter 
this aperture subtends an angle little greater than one minute of arc. 
él 
