stablished that sunlight sterilizes water, for example,
and the quartz mercury-lamp is in daily use for this purpose on a
practicable scale. However, there still appears to be a difference of
opinion as to the destructive effect of radiant energy upon bacteria in
living tissue. It has been shown that the middle ultra-violet rays
destroy animal tissue and, for example, cause eye-cataracts. It appears
possible from some experiments that ultra-violet rays destroy bacteria
in water and on culture plates more effectively in the absence of
visible rays than when these attend the ultra-violet rays as in the case
of sunlight. This is one of the reasons for the use of blue glass in
light-therapy, which isolates the blue, violet, and near ultra-violet
rays from the other visible rays. If the infra-red rays are not
desired they can be readily eliminated by the use of a water-cell.

There is a vast amount of testimony which proves the bactericidal action
of light. Bacteria on the surface of the body are destroyed by
ultra-violet rays. Typhus and tubercle bacilli are destroyed equally
well by the direct rays from the sun and from the electric arcs.
Cultures of diphtheria develop in diffused daylight but are destroyed by
direct sunlight. Lower organisms in water are readily killed by the
radiation from any light-source emitting ultra-violet rays comparable
with those in direct sunlight. From the great amount of data available
it appears reasonable to conclude that radiant energy is a powerful
bactericidal agency but that the action is due chiefly to ultra-violet
rays. It appears also that no bacteria can resist these rays if they are
intense enough and are permitted to play upon the bacteria long enough.
The destruction of these organisms appears to be a phenomenon of
oxidation, for the presence of oxygen appears to be necessary.

The foregoing remarks about the bactericidal action of radiant energy
apply only to bacteria in water, in cultures, and on the surface of the
body. There is much uncertainty as to the ability of radiant energy to
destroy bacteria within living tissue. The active rays cannot penetrate
appreciably into such tissue and many authorities are convinced that no
direct destruction takes place. In fact, it has been stated that the
so-called chemical rays are more destructive to the tissue cells than to
bacteria. Finsen, a pioneer in the use of radiant energy in the
treatment of disease, effected many wonderful cures