ith 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.
Thus, for instance, in the case of the quartz, in the neighbourhood of
a radiation corresponding to a wave-length of 8.5 microns, the
absorption is thirty times greater in the region of the band than in
the neighbouring region, and consequently, after three reflexions,
while the corresponding radiations will not have been weakened, the
neighbouring waves will be so, on the contrary, in the proportion of 1
to 27,000.

With mirrors of rock salt and of sylvine[21] there have been obtained,
by taking an incandescent gas light (Auer) as source, radiations
extending as far as 70 microns; and these last are the greatest
wave-lengths observed in optical phenomena. These radiations are
largely absorbed by the vapour of water, and it is no doubt owing to
this absorption that they are not found in the solar spectrum. On the
other hand, they easily pass through gutta-percha, india-rubber, and
insulating substances in general.

[Footnote 21: A natural chlorate of potassium, generally of volcanic
origin.--ED.]

At the opposite end of the spectrum the knowledge of the ultra-violet
regions has been greatly extended by the researches of Lenard. These
extremely rapid radiations have been shown by that eminent physicist
to occur in the light of the electric sparks which flash between two
metal points, and which are produced by a large induction coil with
condenser and a Wehnelt break. Professor Schumann has succeeded in
photographing them by depositing bromide of silver directly on glass
plates without fixing it with gelatine; and he has, by the same
process, photographed in the spectrum of hydrogen a ray with a
wave-length of only 0.1 micron.

The spectroscope was formed entirely of fluor-spar, and a vacuum had
been created in it, for these radiations are extremely absorbable by
the air.

Notwithstanding the extreme smallness of the luminous wave-lengths, it
has been p