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 possible, after numerous fruitless trials, to obtain stationary waves analogous to those which, in the case of sound, are produced in organ pipes.

The marvellous application M.Lippmann has made of these waves to completely solve the problem of photography in colours is well known.