PROF. P. M. S. BLACKETT delivered the Friday evening discourse on April 20 at the Royal Institution, taking as his subject “Cosmic Rays”. This fascinating subject started more than thirty years ago with the discovery that clean dry air at sea-level is a slight conductor of electricity; it has now grown into one of the important branches of physics, and it perhaps may also be considered as an important branch of astronomy. For whatever the final explanation of the origin of the rays is found to be, it is probable that their origin is of great astronomical significance. The instruments with which the rays have been investigated have been the ionisation chamber, the counter and the cloud chamber, and experiments have been carried out with such apparatus all over the world and at very great heights above the ground and far below the surface. The cosmic radiation is a part, really, of geophysics, to be studied not only in the laboratory but also everywhere that is attainable. It appears from all these results that the earth is being bombarded by streams of positrons and electrons of very great energy. These appear to come continually from outside our galactic system, but from where, or how they are produced, no one knows. The study of the passage of these rays through the atmosphere has led to the discovery of exciting new phenomena. The positron, first detected by Anderson in a cloud photograph, is now known to be one of the main constituents of the rays; and this new member of the group of fundamental particles has very great theoretical interest, since its experimental detection has shown the validity of Dirac's theory of ‘holes’. Very great interest is attached to the behaviour of the very fast cosmic ray particles while passing through matter. The curious and striking phenomenon of the ‘showers’ still awaits explanation. It is clear that one is here in a region of physics where quite new types of phenomena occur.