Several physicists, and in particular Haga, had originally tried to obtain diffraction of X-rays by means of a fine slit, but the rays employed were of too short a wave-length to produce a per ceptible effect. Holweck using radiation of the order of i oo Angstrom units succeeded in demonstrating diffraction by a slit and specular reflection on a metallic mirror.
The very small difference separating the index of refraction of X-rays from unity renders any attempt to apply the ordinary pro cesses of polarisation by reflection very difficult and up till now, the phenomenon of double refraction has not been found. Theory indicates however, that scattering may furnish a method of ob taining linearly polarised radiation. Barkla first showed that a scattering body, composed of a light element, gives rise to a radiation which is practically free from secondary fluorescent radiation. On examining this radiation scattered in a direction at right angles to the incident beam, polarised rays ought to be ob tained, but the polarisation thus obtained, while being perceptible, is very incomplete. Much better results are obtained, as was done by Mark and Szilard, by using crystal diffraction so that the diffracted ray makes a right angle with the incident radiation.
The discovery and study of X-rays has had an especially im portant influence on the development of Physics. It was the properties of conducting gases which led to the discovery of electrons and to that of the radioactive bodies; the diffraction of Röntgen rays by crystals has given new life to crystallography and the study of the whole of the fine structure of matter, at the same time that it enabled X-ray spectra to be obtained. The last through Moseley's law have rendered the unity of structure of the chemi cal elements indisputable and supplied to the Bohr atom its strongest support.
It is the high frequency of Röntgen rays which brings out more clearly the role of light quanta, renders Einstein's law on the photoelectric effect the more easily verified and which permitted the discovery of the Compton effect to be made. Röntgen rays afford ground particularly favourable to the birth of new ideas in the theories of radiation ; ideas which will one day lead to the reunion in the same synthesis of the undulatory and corpuscular aspects of radiation. (DE BR.) X-rays were discovered in the fall of the year 1895 by Pro fessor Wilhelm Konrad Röntgen, professor of physics in the University of Wurzburg, Bavaria. The discovery was the result of a somewhat extensive research concerning the passage of an electric current in an evacuated tube. This subject had received attention for many years and X-rays had unquestionably been produced, though not recognized, by very many investigators. Probably the first person actually so to produce X-rays was William Morgan in the year 1785.
The first recognition of the new rays occurred semi-accident ally : Röntgen happened to notice that when he passed an electric discharge through his tube some crystals of barium-platinocyanide which were in the vicinity became brilliantly fluorescent, although the visible light from the tube was completely screened by black paper. He also found that various substances placed between the tube and a card on which barium-platinocyanide crystals were spread cast a shadow. Röntgen named these new rays, "for the sake of brevity," X-rays. The most striking attribute of the new rays was that they had the power of penetrating objects which were opaque to ordinary light and consequently their potential value in the realm of medicine was immediately recognized. In fact, some four days after the discovery was known in America, X-rays were successfully used to locate a bullet embedded in the calf of a patient's leg.
The tube with which Röntgen made his discovery bears very little resemblance to a modern X-ray tube; it was, in fact, the commonly used. glass bulb called the Crookes tube, in which the cathode stream, instead of being directed on to a metal plate, now called the target, was directed from the cathode or negative electrode on to the glass wall of the tube and produced X-rays at the place of impact.