A crystal of metal consists of molecules, and they of the still smaller atoms. Each atom, in its turn, is made of the still smaller electrons or corpuscles. If these either _are_ light, or are made of even smaller bodies which are, the definition is justified. This is the suggestion, or contention, of Professor Bragg, developed at a recent lecture delivered before the English Royal Institution. Light is regarded as a spreading etheric pulsation, waves in ether. We have it as the visible seven colors from red up to violet, and beyond visibility as the ultra-violet. Still higher etheric pulses, according to the usual theory, are the _x_-rays. Professor Bragg applies his new corpuscular theory to the last alone, though he suggests that it also includes the ultra-violet rays--in which case it must include all the rest. He thinks the _x_-rays corpuscular because of a certain behavior; but the ultra-violet rays have the same behavior--and no one doubts _their_ continuity with the lower rays down to--and far below--the red. What is the behavior on which the argument rests? The term _x_-rays or kathode rays, as popularly used, covers three kinds of emanation in the tube or from radium. The first and grossest ingredient is ordinary matter, whirling atoms of the element helium. The next and finer, the intermediate, is electrons, corpuscles. The third and finest is _x_-rays proper, hitherto considered as merely etheric pulses. Professor Bragg calls them _gamma_ rays, restricting the other term, _x_-rays, for other rays of properties so nearly the same that he includes them in the same argument. When _gamma_ (or _x_-) rays fall on an atom of matter they cause it to discharge one or more of its electrons or corpuscles, the intermediate of the three emanations popularly included under the term _x_-rays. In this connexion they are called _beta_ rays. The professor points out that when _gamma_ (or _x_-) rays produce this discharge from an atom the _beta_ rays to a large degree continue the line of motion of the _gamma_ rays, as if the latter pushed them out of the atoms; and, lastly, that the number of the _beta_ rays depends on the intensity of the _gamma_ rays. The _gamma_ ray, entering an atom, pushes out a corpuscle, a _beta_ ray, and takes its place. It behaves, in fact, as if it were itself a corpuscle, and the word ray is not well descriptive either of it or the _beta_. Nor can it be a mere ether-pulse. The professor suggests that it is a corpuscle, an electron, which has had the ordinary negative charge of electricity proper to electrons neutralized by a positive. Then he proceeds: