The fourth Duke of Edinburgh's Lecture was presented by Professor P. M. S. Blackett, C.H., P.R.S., at the nineteenth Annual General Meeting, held in the Royal Geographical Society's house on 27 October 1965, with the President in the Chair. The President introduced the lecturer as one of the most eminent physicists of our time. Over the last thirty years he had held a number of very distinguished appointments, and was until recently Professor of Physics at the Imperial College of Science and Technology. He is now deputy chairman of the Ministry of Technology's Advisory Council. In 1948 Professor Blackett was awarded the Nobel Prize for Physics. A summary of the main points covered is given below.