.
In the days of the carbon filament lamp the 16-candle-power lamp was
used. Certainly the heating effect has advantages in some cases over
other methods of heating. The light-rays penetrate the tissue and are
absorbed and transformed into heat. Other methods involve conduction of
heat from the hot air or other hot applications. Of course, it is also
contended that the light-rays are directly beneficial.

Light is also concentrated upon the body by means of lenses and mirrors.
For this purpose the sun, the arc, the quartz mercury-arc, and the
incandescent lamp have been used. Besides these, vacuum-tube discharges
and sparks have been utilized as sources for radiant energy and
"electrical" treatment. Roentgen rays and radium have also figured in
recent years in the treatment of disease.

The quartz mercury-arc has been extensively used in the past decade for
the treatment of skin diseases and there appears to be less uncertainty
about the efficacy of radiant energy for the treatment of surface
diseases than of others. Herod related that the Egyptians treated
patients by exposure to direct sunlight and throughout the centuries and
among all types of civilization sunlight has been recognized as having
certain valuable healing or purifying properties. Finsen in his early
experiments cured a case of lupus, a tuberculous skin disease, by means
of the visible and near ultra-violet rays in sunlight. He demonstrated
that these were the effective rays by using only the radiant energy
which passed through a water-cell made by using a convex lens for each
end of the cell and filling the intervening space with water. This was
really a lens made of glass and water. The glass absorbed the
ultra-violet rays of shorter wave-length and the water absorbed the
infra-red rays. Thus he was able to concentrate upon the diseased skin
radiant energy consisting of visible and near ultra-violet rays.

The encouraging results which Finsen obtained in the treatment of skin
diseases led him to become independent of sunlight by equipping a
special arc-lamp with quartz lenses. This gave him a powerful source of
so-called chemical rays, which could be concentrated wherever desired.
However, when science contributed the mercury-vapor arc, developments
were immediately begun which aimed to utilize this artificial source of
steady powerful ultra-violet rays in light-therapy. As a consequence,
there are now available very compact quartz mercury-arcs desi