waves of still greater wave-length
are due to the slower oscillations in certain electric circuits
caused by lightning discharges, etc.

The Roentgen rays were discovered by Roentgen in 1896 and they have been
studied and applied very widely ever since. Their great use has been in
X-ray photography, but they are also being used in therapeutics. The
extreme ultra-violet rays are not available in sunlight and are
available only near a source rich in ultra-violet rays, such as the
arc-lamps. They are absorbed by air, so that they are studied in a
vacuum. These are the rays which convert oxygen into ozone because the
former strongly absorbs them. The middle ultra-violet rays are not found
in sunlight, because they are absorbed by the atmosphere. They are also
absorbed by ordinary glass but are freely transmitted by quartz. The
nearer ultra-violet rays are found in sunlight and in most artificial
illuminants and are transmitted by ordinary glass. Next to this region
is the visible spectrum with the various colors, from violet to red,
induced by radiant energy of increasing wave-length. The infra-red rays
are sometimes called heat-rays, but all radiant energy may be converted
into heat. Various substances transmit and absorb these rays in general
quite differently from the visible rays. Water is opaque to most of the
infra-red rays. Next there is a region of wave-lengths or frequencies
for which no radiant energy has been found. The so-called electric waves
vary in wave-length over a great range and they include those employed
in wireless telegraphy. All these radiations are of the same general
character, consisting of electromagnetic energy, but differing in
wave-length or frequency of vibration and also in their effects. In
effect they may overlap in many cases and the whole is a chaos if the
physical details of quantity and wave-length are not specified in
experimental work.

[Illustration: In art work

In a haberdashery

JUDGING COLOR UNDER ARTIFICIAL DAYLIGHT]

[Illustration: In an underground tunnel

In an art gallery

ARTIFICIAL DAYLIGHT]

It has been conclusively shown that radiant energy kills bacteria. The
early experiments were made with sunlight and the destruction of
micro-organisms is generally attributed to the so-called chemical rays,
namely, the blue, violet, and ultra-violet rays. It appears in general
that the middle ultra-violet rays are the most powerful destroyers. It
is certainly e