The present X-ray tube has been evolved from the older Oeissler or vacuum tube. In the latter, when the pressure is reduced slightly, the resistance to electric discharge is lessened, and discharge takes place through the tube with striking phenomena of illumination, depending in character on the degree of exhaustion. The chemical nature of the residual gas also affects the character of the illumination; but we are concerned only with the presence of ordinary air. Exhaustion beyond a certain degree increases the resistance to electric discharge through the tube; and as the resistance increases, the luminosity of discharge disappears. But at a point of high exhaustion a fluorescence becomes evident on the walls of the tube, or on any object interposed between the electrodes. In vacuum tubes made of soda glass this fluorescence is of a transparent apple-green colour; with lead glass it is of a bluish tinge.

Sir William Crookes, about 1891, studied and interpreted the phenomena of such discharge in high vacuum tubes, the pressure in the tubes he worked with being reduced to about
one millionth part of an atmosphere. He shewed the fluorescence to be due to a bombardment of the interposed object by streams of negatively charged particles, moving from the kathode of the tube at a very high velocity. Such particles are now spoken of as electrons. In tubes such as were experimented on by Crookes these electrons are considered to move with a velocity equal to about one-tenth the velocity of light.

Lenard, workmg later with highly exhausted Crookes tubes, named this stream of electrons ‘ kathode rays,’ and found that those rays also existed outside the tube. He shewed that they could pass through some substances opaque to ordinary light, could excite fluorescence on suitable substances, and could act on sensitive photographic plates.

Roentgen, one year later—about the end of 1895—discovered that along with those kathode rays proper there were emitted by such high-vacuum tubes other rays having some differentiating properties. These he named X rays.

One essential physical difference consists in the failure of a magnet to deflect these X rays, whilst kathode rays are so deflected; nor can X rays be either reflected or refracted. Thus, in contrast to the kathode rays, which consist of what may be termed a corpuscular stream, X rays are true ethereal rays.

These X rays are produced by the impingement of the rapidly moving kathode rays on any obstacle in their path. In the earliest 4ubes, as represented in Fig. 1, they were produced on the glass wall of the tube wherever it was struck by the rays proceeding from the kathode. That was in the form of a flat disc, whilst the anode was annular in form.

Herbert Jackson made important and essential alterations in design, and on his plan are constructed all X-ray tubes of the present day. The kathode was by him made of concave form, so as to focus the rays proceeding from it to the centre of the tube, and near that focal point was introduced a metallic target called the anode or antikathode. This target is set at an angle of 45 degrees to the axis of the tube, so as to throw the main part of the X rays to one side of the tube. This device rendered the study and employment of X-ray effects much more precise, since the rays proceed from a definite point or small area of the target.

Of the physical properties of X rays little need be said. The property upon which depends their use in medicine as a diagnostic aid is that of penetrating many substances opaque to ordinary light. The degree of penetration varies inversely with the density of the opposing substance. Thus, differentiated shadows are cast of the different tissues, and departures from the normal may be detected. Bone, being denser than muscular or other soft tissue, offers greater resistance to the rays; hence it casts a deeper shadow, and alterations in its density, as in necrosis, will influence the shadow cast, while such lesions as fracture will do so more markedly.

But to the naked eye X rays are not visible, and such interference with them not discernible ; hence two other properties are brought into requisition—that of rendering fluorescent certain substances such as the platino-cyanides of barium and potassium, and that of acting upon sensitised photographic plates.

The intensity and differentiation of the shadow cast on a fluorescent screen by a certain radiation will depend upon the nature of the body or substance interposed between the screen and the source of the transilluminating rays; and so likewise will depend the image impressed on a photographic plate exposed for a suitable time.

The effect of X rays on living tissue exposed to them is  discussed in the section on ‘ Therapeutics.’

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