X-RAYS. 690 X-BAYS. (6) Diffusion. It has been stated above that X-rays do not experience ordinary relleetion, but many experiments have sliown that they are able to penetrate into regions beliind obstacles which are oijaque to them and where they could not come by any direct path. This led many ob- servers to believe that the rays were reflected dif- fusely in the same manner that ordinary sunlight is reflected from a white wall. The true ex- planation, however, seems to be that under the action of X-rays those bodies which absorb them hae the power of emitting others, so that they become, as it were, new sources of X-raj's. This is called 'secondary radiation,' and plays an im- portant part in practical applications of X-rays to surgery. When Rontgen made his first observations on X-rays there seemed no obvious explanation of these varied phenomena. They could not be transverse waves, because they did not exhibit polarization. The possibility of there being waves at all was not disproved at first owing to the lack of knowledge in the first few months in re- gard to interference and diffraction ; and for some time the prevalent idea was that they were due to longitudinal waves in the ether. Another explanation, however, which was much more satis- factory, and which explains all the phenomena, was advanced almost simultaneously by Profes- sors Stokes and Thomson of the University of Cambridge, England, and by Professor Lehmann of Karlsruhe, Germany. According to this, X-rays are disturbances in the luminiferoiis ether of the nature of wave-motion, but not con- sisting of t ruins of iraics. It is exactly as in the similar jilienomena of sound. A musical note is due to a long continued train of waves pro- duced by the regular vibrations of some musical instrument, while a noise is produced by a series of short disconnected pulses in the air. It was shown mathematically by Stokes that pulses, or extremely short disturbances, in the ether would travel in straight lines and would not exhibit refraction, diffraction, polarization, or interfer- ence. The explanation of the origin of these pulses is extremely simple. The cathode rays are known to consist of streams of minute ma- terial particles which are electrically charged and which are moving at a rate as great in many cases as 40,000 miles a second. If these rapidly moving electrified bodie-s are suddenly brought to rest by impact on a solid wall, there will be necessarily a great change in the electrical and magnetic properties of the ether at the point w-here one of the particles of the cathode rays strikes the wall, and tliis point will therefore serve as a centre of disturbance from which a pulse will be sent out into the ether. It follows also that, since there is no regularity in the cathode rays, there can be no regularity in the emission of these pulses. It is thus seen that all the main properties of the X-rays, at least from a physical point of view, are explained. BiULiOGBAPiiY. All of Riintgen's original papers, together with extracts from the papers of Stokes and .1. J. Thomson, will be found in Barker, Jiiintqen Rays, in "Scientific Jlcmoir Series," vol. iii. (New York, 1890). The various scientific journals and Prncecditrijs of learned so- cieties also contain a record of many researches in this field. See PADlOACTiviTy. X-Rays in Medicine and Slrgert. The use- fulness of this radioactivity to the physician depends on the power of the X-rays to pene- trate various substances of different density and opacity, and the absorptive power of the different tissues of the body. X-ray pictures of parts of the body as seen on the screen, or when photo- graphed, are due to the fact that substances of different chemical composition, molecular group- ing, and thickness absorb different amounts of the rays. In experiments or observations with the X-rays the physician needs the following outfit: a static machine (or an induction coil), a vacuum tube and holder, and a fluorescent screen or a fluoroscope. In order to preserve records of his work, he will need, further, a photographic outfit. During an X-ray examination the pa- tient must be properly supported. He must either lie flat on a canvas stretcher or be seated on a high stool with his back against a vertical canvas support. The sitting or the recumbent position is chosen in accordance with the part of the body to be examined. In certain cases he may be examined standing. Examinations with a fluoroscope or screen must be made in a dark room, and the physician must prepare his eyes for the work by previously spending about ten minutes in the dark, or by wearing dark glasses for about twenty minutes before entering the room. At first the operator sees only the green light in the tube: but shortly his retina> begin to recognize objects in the room. An open fluores- cent screen may be used, with a dark cloth cover- ing it and the physician's head, or the fluoroscope nuiy be held, as in using a stereoscope, against the body of the patient. If a radiograph is to be taken, all metal (such as buckles and buttons) and all bone articles, glass, etc., must be removed from the clothing of the patient. While ordinary dressings of gauze and cotton covering a wound do not interfere, iodoform, plaster of Paris, ad- hesive plaster, and ordinary wooden splints cast shadows that obscure the results. It is cus- tomary to take two views of fractures, new growths, and foreign bodies, from different points, to avoid errors. The physician must first study radiographs of normal structures under normal conditions, be- fore he can hope to interpret properly X-ray appearances on the screen or when photographed. When proficient, the observer may, in most cases, recognize the presence and extent of tuberculosis, pneumonia, pleurisy, hydrothorax, emphysema, and empyema, and the presence of bronchitis may be inferred in some instances from the shortened excursion of the diaphragm and the restricted movement of the lower part of the chest, as well as from collections of secretion when f(nind. Much can be juilged concerning the heart, ami the inac- curacy of percussion may be corrected, as to size and jiosition. If tlic heart be dis|)laced, as e.g. by fluid or gas in the pleural cavities, pneumonia, pleurisy, tuberculosis, aneurism or tiunor, or if the heart be attached or unusually placed or malformed, the condition may be ascertained by the X-ray examination. Effects of treatment of the organ may be watched. .Aneurisms may be clearly defincci in many cases, or recognized when no other metliod is of service. .Ml new L'rowths as well as enlarged glands within (lie thorax are within the scope of the diagnosing Xray. Similar though less complete information may lie gained concerning the a'sophagus, stomach, and
