He had
remarked that, in their study, the difficulty of research proceeds
from the fact that the extreme waves of the infra-red spectrum only
contain a small part of the total energy emitted by an incandescent
body; so that if, for the purpose of study, they are further dispersed
by a prism or a grating, the intensity at any one point becomes so
slight as to be no longer observable. His original idea was to obtain,
without prism or grating, a homogeneous pencil of great wave-length
sufficiently intense to be examined. For this purpose the radiant
source used was a strip of platinum covered with fluorine or powdered
quartz, which emits numerous radiations close to two bands of linear
absorption in the absorption spectra of fluorine and quartz, one of
which is situated in the infra-red. The radiations thus emitted are
several times reflected on fluorine or on quartz, as the case may be;
and as, in proximity to the bands, the absorption is of the order of
that of metallic bodies for luminous rays, we no longer meet in the
pencil several times reflected or in the rays _remaining_ after this
kind of filtration, with any but radiations of great wave-length.
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