The ultraviolet absorption spectrum of vitamin A with itsmaximum near 328 m,. was described nearly 20 years ago (Morton & Heilbron, 1928a, b). The intensity of absorption shown at that wave length by a fish-liver oil or concentrate was conveniently expressed in terms of E "lo 328 m,u. (Gillam & Morton, 1929), and this magnitude was early seen to afford a guide to potency. If a number of fish-li...