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.
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