THE study of the oil-soluble vitamins has supplied overwhelming proof that both provitamin D and the antirachitic vitamin itself exhibit photochemical properties. As regards the action of light on vitamin A, the balance of opinion has, we believe, hitherto been in favour of an indirect oxidation process rather than a purely photochemical mechanism. Although the evidence for the former has been somewhat ill-founded, its adoption has led to the neglect of the latter view, which we are now able definitely to prove is the correct one.