The present advances in the phototherapy of rickets, tuberculosis and other pathologic conditions, together with the recent laboratory observations concerning the physiologic effects of light, have established conclusively that ultraviolet rays produce profound changes in the blood, the metabolism and other physiologic functions of the body. Furthermore, any one at all familiar with the work done along these lines will agree at once that such effects as have been noted and described cannot be ascribed to a "reflex" reaction produced by the sunburn or to irritation of the superficial layers of the epidermis. On the contrary, the striking effects which have been noted render almost inevitable the assumption that the effective ultraviolet rays of energy must penetrate at least as far as the superficial blood circulation in order to produce the various chemical changes which have been repeatedly described. It is therefore surprising to find that the literature on the