174 8S. P. Langley—Selective Absorption of Solar Energy. 
limit then assigned to the normal spectrum by experiment. 
The writer’s further investigations, however, led him to b 
lieve that this immense and unverified extension really ex- 
isted, and to thus confirm by independent means the statements 
of Tyndall and others, as to the great heat in this region. He 
was unable to determine its exact limit with the grating as then 
used, on account of the over-lapping spectra, but was, some two 
years since, led, from experiments not here detailed, to suspect 
the existence of solar heat at a distance of nearly four times the 
wave-length of the lowest visible line, A(A=0"-76) or at A=3!"0. 
We receive all the solar radiations through an absorbing 
atmosphere, and it is of the first consequence to determine the 
rate of this selective absorption for each separate ray. This has 
(owing to the difficulties before alluded to) never yet been, s0 
far as I know, attempted. It forms a prominent part of the 
present design. 
e great difficulty in this investigation, after the provision 
of a sufficiently delicate heat-measurer, lies in the varyl 
amount of radiant energy which our atmosphere transmits, evel 
for equal air-masses. ‘The solar radiation is itself sensibly con- 
stant, but the variations in the radiant heat actually transmitted 
are notable, even from one minute to another under an appal- 
ently clear sky. The bolometer, in fact, constantly sees (if I 
may use the expression) clouds which the eye does not. That 
he variations from minute to minute (under a visually clear 
sky) amount frequently to ten times the probable instrumental 
an 
afternoon. ven of the twenty-nine days cited, and which 
may be considered exceptionally fair, it will be seen that 
of measures throughout the spectrum daily, one when the rays 
