It is generally conceded that the study of radiant energy as such is a matter involving the work of the physicist. It is to their credit that the various forms of radiant energy have been explored, measured, and described. Of the new aids which physics has given workers in biology, probably the most important are the quantum theory of light, and the new implements, techniques of generating, manipulating and measuring radiations. Radiant energy of desired wave lengths and frequencies is now at the command of research workers in the field of biology and physiology who are engaged in the study of the effects of radiant energy on the living organism.