ed, in 1886,[737] by
further particulars learned with the help of rock-salt lenses and
prisms, glass being impervious to very slow, as to very rapid
vibrations. Traces were thus detected of solar heat distributed into
bands of transmission alternating with bands of atmospheric absorption,
far beyond the measurable limit of 5.3 microns.

In 1894, Langley described at the Oxford Meeting of the British
Association[738] his new "bolographic" researches, in which the
sensitive plate was substituted for the eye in recording deflections of
the galvanometer responding to variations of invisible heat. Finally, in
1901,[739] he embodied in a splendid map of the infra-red spectrum 740
absorption-lines of determinate wave-lengths, ranging from 0.76 to 5.3
microns. Their chemical origin, indeed, remains almost entirely unknown,
no extensive investigations having yet been undertaken of the slower
vibrations distinctive of particular substances; but there is evidence
that seven of the nine great bands crossing the "new spectrum" (as
Langley calls it)[740] are telluric, and subject to seasonal change.
Here, then, he thought, might eventually be found a sure standing-ground
for vitally important previsions of famines, droughts, and
bonanza-crops.

Atmospheric absorption had never before been studied with such precision
as it was by Langley on Mount Whitney. Aided by simultaneous
observations from Lone Pine, at the foot of the Sierra, he was able to
calculate the intensity belonging to each ray before entering the
earth's gaseous envelope--in other words, to construct an
extra-atmospheric curve of energy in the spectrum. The result showed
that the blue end suffered far more than the red, absorption varying
inversely as wave-length. This property of stopping predominantly the
quicker vibrations is shared, as both Vogel and Langley[741] have
conclusively shown, by the solar atmosphere. The effect of this double
absorption is as if two plates of reddish glass were interposed between
us and the sun, the withdrawal of which would leave his orb, not only
three or four times more brilliant, but in colour distinctly
greenish-blue.[742]

The fact of the uncovered sun being _blue_ has an important bearing upon
the question of his temperature, to afford a somewhat more secure answer
to which was the ultimate object of Professor Langley's persevering
researches; for it is well known that as bodies grow hotter, the
proportionate representation in