The stars everlasting,
Are fugitive also,
And emulate, vaulted,
The lambent heat-lightning
And fire-fly’s flight.”
ROENTGEN’S X RAYS.
Fig. 232a.—Living Hand. Exposure, 4 Minutes.
On page [507] reference will be made to certain remarkable effects observed by Mr. Crookes when the electric discharges from an induction coil are passed through very highly exhausted tubes. These phosphorescent and mechanical effects Mr. Crookes attributed to streams of “radiant matter” shot off from the negative pole with immense velocities—the matter not being that of the electrode itself, but particles of the extremely rarefied residual gas, which, being comparatively few, could mostly traverse the tube in straight lines without coming into collision with their fellows, and thus a class of phenomena, different from the striated discharges in the ordinary and less highly exhausted Geissler tubes, comes into view. The emanations from the negative pole, or cathode, in highly rarefied gases became known as the “cathode rays,” and they began to be further examined by other observers, and more particularly in 1894 by Hittorf, and by M. Lenard, a Hungarian physicist, who found that they pass through thin plates of metal, and through wood and other substances not transparent to ordinary light. It was also observed by Lenard, and also previously by Hertz, that there are several kinds of cathode rays, which differ from each other as regards their powers of exciting phosphorescence, capability of being deflected by a magnet, and the degrees in which they are absorbed by various media. But universal attention was drawn to this subject by the announcement, at the end of 1895, of certain discoveries made by Dr. W. K. Roentgen, a professor of physics at Wurzburg. He covered a highly-exhausted Crookes’ tube with black cardboard, and found that when the discharge of a large induction coil was passed through the tube in a dark room, a piece of paper coated on one side with platino-cyanide of barium, and held near the covered tube, glowed with a brilliant fluorescence, no matter which side of the paper was turned towards the tube; and even at a distance of two yards some fluorescence was still visible. On experimenting with various bodies interposed between the covered tube and the fluorescent screen, it was found that the emanations passed through nearly every substance with more or less facility. The screen lit up when placed behind a book of a thousand pages, also behind two packs of cards. A single layer of tin-foil scarcely threw a shadow, and several thicknesses were required to produce a distinct effect. Deal boards, an inch thick, offered little resistance. A very thick plate of aluminium (6
10 inch) reduced the fluorescence, but still allowed some rays to pass. The hand held before the fluorescent screen showed a dark shadow of the bones only, with but a faint outline of their fleshy investment. Copper, silver, gold, platinum, and lead, in comparatively small thicknesses, intercept these rays. Thus a plate of lead only five hundredths of an inch thick almost stops them. Increase of thickness increases the resistance to their passage in all cases; but the comparative transparency of a body cannot be deduced from its thickness and density. Many other bodies besides platino-cyanide of barium become fluorescent under the influence of these rays, such as certain kinds of glass, Iceland spar, rock-salt, etc. Dr. Roentgen is convinced that these rays are not the cathode rays or any part of them; but as the theoretical nature of the new rays has not yet been explained, he has preferred to provisionally call them the X rays, a denomination doubtless suggested by the use of the symbol x in algebra to represent unknown quantities.
The source of the X rays, Roentgen states, is at the place where the cathode rays strike the walls of the exhausted tube, and produce the most brilliant phosphorescence; but they cannot be cathode rays which have merely passed through the glass, for, contrary to what has been observed with respect to the latter, they cannot be deflected by a magnet. Nor is glass the only substance in which they can be generated, for they were obtained from an apparatus in which the cathode rays were made to impinge upon a plate of aluminium nearly one-tenth of an inch thick. Photographic dry plates are also sensitive to the X rays, and their power to pass through wood, ebonite, etc., makes the experiments of testing the opacity, or otherwise, of various objects for them quite easy. It is necessary merely to place the object on the closed cover of the dark slide, and place the whole under the vacuum tube; all the exposure, which is somewhat prolonged in most cases, may be made in ordinary light. But the light-tight boxes, in which photographic plates are packed, cannot, of course, be brought near the apparatus, as they are completely permeable to the X rays, and their whole contents may be rendered useless. The impression obtained on the photographic plate is not so much a photograph as a shadow of the interposed object—a shadow more or less dense in the positive print according to the permeability of the object, and the length of the exposure. These photographic results have sometimes been called “shadowgrams,” “radiograms,” “radiographs,” “skiagraphs,” etc. The word skiagraph appears the most appropriate designation. That the emanations from the phosphorescing substance on which the cathode rays impinge are entitled to be also called rays, appears from the regularity of the shadows thrown on the fluorescent screen or photographic plate; and the fact of their propagation in straight lines was proved by Dr. Roentgen obtaining a pin-hole photograph of the phosphorescing part of the vacuum tube, when the latter was enveloped in black paper. Why a pin-hole and not a lens was used for taking this photograph will presently appear.