FOR many years science has been investigating the physiological effects of radiant energy with the result that some definite knowledge is now available. However, almost every question that is asked pertaining to some phase of radiation therapy is wholly unanswerable, or incompletely answerable at best. Seldom does the public "keep up with science" but occasionally it "gets ahead of knowledge." People have knowingly or unknowingly practiced radiation therapy with solar energy since the dawn of savagery. Notwithstanding all this, the entire field is still in the "twilight zone of knowledge." Combining this with the natural desire to grasp at any promising boon to life and to health we have a situation which is fraught with some dangers and many difficulties.
There are uncounted photochemical reactions. Doubtless radiant energy of all wave-lengths in the ultraviolet region plays many parts in chemical processes. It is possible that radiant energy of all these wave-lengths may eventually be used in some manner to benefit the life and the health of human beings. However, we are far from a complete knowledge of these possibilities. Interest in ultraviolet therapy at present is chiefly concentrated in the spectral region between the wave-length limits from 3200 A and 28ooA. Ultraviolet energy of other wave-lengths produces many effects and, therefore, has or will have many practical applications. However, for such purposes as the prevention and cure of rickets, radiations of shorter wave-lengths than 32ooA seem to be the most effective. There are some indications that radiant energy of shorter wave-length than 28ooA, or thereabouts, may be harmful, at least in overdoses. In the absence of complete knowledge it is prudent to avoid the use of radiant