Two years ago, Dr. Spear delighted us by recaling Douglas Lea, friend, colleague and fellow member of the H.P.A., with a vividness which I thought could hardly be surpassed. Moreover, he commented upon Lea's scientific work with such insight and true appreciation that the lecture constitutes to my mind a most fitting and perfect memorial. Though I find it hard to add substantially to the portrait Dr. Spear painted for us then, I do welcome this opportunity of acknowledging the great debt I owe to Douglas Lea. I was privileged to know him intimately. We were both at Trinity and we were both research students in the Cavendish Laboratory during that exciting period in physics from 1924 to 1933, during which Cockcroft and Walton first achieved nuclear disintegration by accelerated particles, Anderson discovered the positron, Chadwick discovered the neutron, and the Curies discovered artificial radioactivity. Rutherford's genius pervaded the Cavendish in those days, and I think it may be said that in the directness of his approach to problems in radiobiology, Lea showed himself a worthy student of a great master.Lea and I differed in age by four years—which academically amounts to more than a generation—so that I have no very vivid personal recollections of him as an undergraduate. This is a gap in our record of Lea which we may hope some future lecturer will fill in.