The interest in the long assumed subtle health-giving virtues of sunshine has been given a remarkable impetus by the convincing discovery that sunlight has demonstrable curative powers as well as prophylactic potencies in relation to rickets. It is not always possible, however, to rely for protection on sunshine or on that diffuse sunlight known as skyshine. When heliotherapy has become impossible or inadvisable, resort is now successfully made to artificial ultraviolet radiations, with noteworthy success. In fact, Hess1 has asserted that ultraviolet irradiation is a far more potent and reliable specific than cod liver oil in rickets. Treatment with ultraviolet rays has come to include the use of irradiated foods and irradiated ergosterol (viosterol). Without minimizing these demonstrations, however, it must be emphatically stated that radiant energy will not accomplish the desired physiologic effect regardless of the diet.
Perhaps the most general statement of the potency of irradiation is