cts of light at the present time are
conflicting and they do not rest upon an established scientific
foundation. Furthermore some of them are at variance with the
possibilities and an unprejudiced observer must conclude that much
systematic work must be done before order may arise from the present
chaos. This does not mean that many of the effects are not real, for
radiant energy is known to cause certain effects, and viewing the
subject broadly it appears that light is already serving humanity in
this field and that its future is promising.

The present lack of definite data pertaining to the effects of radiation
is due to the failure of most investigators to determine accurately the
quantities and wave-lengths of the rays involved. For example, it is
easy to err by attributing an effect to visible rays when the effect may
be caused by accompanying invisible rays. Furthermore, it may be
possible that certain rays counteract or aid the effective rays without
being effective alone. In other words, the physical measurements have
been neglected notwithstanding the fact that they are generally more
easily made than the determinations of curative effects or of germicidal
action. Radiant energy of all kinds and wave-lengths has played a part
in therapeutics, so it is of interest to indicate them according to
wave-length or frequency. These groups vary in range of wave-length, but
the actual intervals are not particularly of interest here. Beginning
with radiant energy of highest frequencies of vibration and shortest
wave-lengths, the following groups and subgroups are given in their
order of increasing wave-length:

Roentgen or X-rays, which pass readily through many substances
opaque to ordinary light-rays.

Ultra-violet rays, which are divided empirically into three
groups, designated as "extreme," "middle," and "near" in
accordance with their location in respect to the visible
region.

Visible rays producing various sensations of color, such as
violet, blue, green, yellow, orange, and red.

Infra-red or the invisible rays bordering on the red rays.

An unknown, unmeasured, or unfilled region between the
infra-red and the "electric" waves.

Electric waves, which include a class of electromagnetic
radiant energy of long wave-length. Of these the Herzian waves
are of the shortest wave-length and these are followed by
"wireless" waves. Electric