IN his Friday evening discourse at the Royal Institution on January 29, Sir William Bragg discussed recent work in crystallography. When X-rays were first applied to the determination of crystal structure, the forms examined were naturally those of simple design. As the technique improved, and as insight was gained into the modes of construction of which Nature made the most use, it became possible to attack more difficult examples with success. Quite recently, the X-ray methods have been able to give material assistance in the examination of the complicated bodies which play the leading part in the living organism, such as the proteins. X-rays have the special power of discerning regularities in the arrangement of the atoms and molecules of which substances are built. Until they were applied to this purpose, no one had suspected how usual and fundamental such regularities were. Nature's structures are generally based on the repetition of some unit of pattern. Even a very small crystal is formed of the orderly repetition of some atomic design repeated billions of times: and the minute but ubiquitous proteins of all living organisms possess this ordered arrangement though they are far too small to be seen in the microscope.