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 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