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1. Holder containing highly sensitized photographic plate. 2. Pencil, of which shadow print is desired. 3. Crookes tube. 4. Cathode or negative pole. 5. Positive pole. 6. Induction coil giving very high potential discharges.

that the rays fell upon the glass plate at an angle of 45 degrees, and thus were reflected into the tube in a direction parallel to its axis. Two per cent of the direct rays are estimated to have been reflected. The possibility of reflecting X rays has also been demonstrated by Messrs. McLennan, Wright, and Keele of the University of Toronto (see below).

Röntgen himself cautiously hints that the X rays may be physic ally dissimilar to ordinary light in any of its known forms, visible or invisible. He leans to the opinion that they are due to longitudinal or condensational vibrations of the ether, instead of to the transversal or distortional vibrations which are the fundamental assumption of the existing theory of light. For it should be noted, that, although variations in the line of propagation of the ray have no place in the accepted explanations of the nature of light, their existence has been admitted as possible, and even insisted on as actual, by physicists, notably Lord Kelvin. The fact that X rays excite fluorescence-a phenomenon recognized as the degradation of short waves into longer ones-is compatible with the theory that they are transverse vibrations of very short period, but it does not exclude the hypothesis that we are dealing with longitudinal vibrations which, so far as we know, may have the power of setting up transverse vibrations affecting the eye. Air is exceedingly opaque to transverse waves of very short period, but these X rays are operative after traversing long distances through air. Tesla claims to have observed their effect at a distance of over forty feet. This, again, would seem to distinguish them from ultra-violet rays; but there is such a thing as selective absorption; and air may absorb short waves up to a certain point, yet be transparent to waves shorter still.

We have already mentioned that Iceland spar has probably been found capable of doubly refracting the X rays, so that Röntgen's conclusion that the rays cannot be polarized may require to be modified. It is, however, singular to note that the rays seem to pass with equal facility through two plates of tourmaline, whether their axes be parallel or at right angles. This is a very important observation, since the phenomena of polarization really form the corner-stone of the theory that light depends upon vibrations transverse to the direction of propagation. Yet even here there is the disturbing possibility

that analogy may not hold when we come to deal with transverse vibrations of a period comparable to molecular dimensions.

In the present state of our knowledge, no conclusive argument can be made for the hypothesis either of longitudinal or of excessively short transverse etheric vibrations.

Other hypotheses as to the nature of the X rays have been advanced, which we indicate briefly as follows:

Some investigators hold them to be identical with the cathode rays of Hertz and Lenard. Röntgen, however, points out that the X rays, unlike the cathode rays, are not deflected by a magnet; they are not reflected by a mirror of polished steel; they suffer far less absorption and diffusion than the latter: they have a much greater penetrative power: 6 or 8 centimetres seems to be the limit of the "radiation length" of cathode rays in air at atmospheric pressure, whereas Tesla has shown X rays to be operative at a distance of over 40 feet; cathode rays will pass through only thin films of soap, aluminium foil, etc., while Edison has shown X rays to be instantly operative through eight inches of wood; and, finally, Röntgen claims that while the cathode rays emanate directly from the cathode, the starting point of the X rays is the luminescent spot on the glass wall of the discharge tube at which the cathode rays terminate.

The points of similarity between X and cathode rays are their actinic properties, their rectilinear propagation, and the fact that in both cases the relative transparency of bodies to them seems to depend upon the conditions of varying density or atomic weight. Some authorities, it should be noted, as Professor Rowland of Johns Hopkins University, incline to the view that the source of the X rays is to be found at the anode rather than the cathode.

Others, including Tesla, suggest that the so called X rays, as well as the cathode rays, are not rays (i. e., etheric waves) at all, but streams of particles of ordinary matter shot forth, as it were (or repelled), from the cathode surface, and of sufficient minuteness and momentum to traverse the interspaces between the molecules in the wall of the tube and other solid bodies. On the other hand, Professor A. W. Wright of Yale thinks the X rays may be the ordinary cathode rays filtered, so to speak, by passing through the glass, of the material particles projected from the electrode.

Mr. S. H. Emmens of New York city maintains that the X rays necessitate no new hypothesis: they are not limited to emissions from electrified vacuum tubes, but exist in sunlight, and in radiations from lamps, electric ares and in fact all sources of radiant energy. Assisted by his son, he claims to have obtained X-ray pictures of objects placed upon a sensitized plate excluded from sunlight by a vulcanite screen, simply as an effect of long exposure to the penetrative rays in the sunbeams. He also claims to have obtained similar effects in darkness as a result of the invisible X radiance to be found everywhere. His conclusions are, however, open to question, as sunlight is known to penetrate wood, ebonite, and other substances, and there are different ways in which the photography of obscure objects can be obtained.

Ever since the first experiments confirmatory in general of the observations of Dr. Röntgen, efforts have been directed toward finding some means of intensifying the effect of the X rays, so as to shorten the requisite time of exposure and make possible greater distinctness in the radiotypes. To this end a most important advance-in many respects the most important discovery made since Röntgen's original announcement-was made on February 11 by Mr. J. C. MacLennan, an assistant to President Loudon in the physical laboratory of the University of Toronto, in conjunction with Mr. C. H. C. Wright and Mr. Keele of the School of Practical Science, Toronto. By means of a glass bell-jar held over a Crookes tube, they succeeded in obtaining reflection of the X rays, and practically demonstrating the possibility of focusing them. Later the same investigators succeeded in obtaining shad

ow pictures through several folds of black paper after an exposure of only one second-practically showing the possibility of instantaneous results. The experiments are described by Mr. MacLennan, in substance as follows:

"In order to determine whether the rays could be reflected, a surface of clean mercury was prepared, and it was found that when the rays were directed toward it, sensitized films protected from direct radiation were fogged by some action coming from the mercury. To test this apparent reflection still further, a sensitized film, protected by a plate-holder, was placed at a distance of about twenty centime tres below the Crookes tube. A thick plate of glass was then inserted midway between the tube and the film, parallel with the latter, with the intention of screening the plate in part from the action of the rays. The tube was then excited for some time; and, on developing the film, it was found that the rays evidently travelled in straight lines, since the part of the film protected by the glass plate was well defined and entirely unaffected by them. This experiment was repeated, the arrangement of apparatus being identical, with the soie exception that a glass bell-jar was placed over the whole. Develop ment of the film in this case showed (1) no action on the film outside the jar; (2) no indication of the interposed glass plate acting as a screen; (3) the action much more intense than in the previous experiment, proving conclusively the reflection of the rays from the surface of the jar.



By the employment of this method the time of exposure was reduced on February 11 almost to instantaneousness.' A shadow print of a medal placed within a wooden leather-covered jewel case was obtained by an exposure of four and a half seconds. Very good results were similarly obtained by an exposure of one second through five folds of black paper.

About the same time Dr. W. J. Morton of New York city also demonstrated the possibility of effecting something like a concentration of the X rays, by means of a vacuum tube having its negative electrode fixed upon the outside of the bulb. A conical stream of cathode rays proceeded from the concave surface of the negative terminal against the opposite side of the bulb, there setting up a powerful fluorescence. Dr. Morton has the honor of being the first in America to use the disruptive discharges of static electricity in producing X rays, and the first also to demonstrate that radiotypes can be taken by means of other than Crookes tubes. Professor A. W. Wright of the Sloane physical laboratory at Yale, is said to have been the first in America to confirm the reported existence of X rays.

Various instruments have already been devised for rendering the shadow pictures instantly visible, and thus obviating the necessity of awaiting the comparatively slow process of photographic development. In all cases the same principle is employed the throwing of a shadow upon a fluorescent screen instead of a photographic plate.



The earliest reported device of this sort is the "cryptoscope" invented by Professor Salvioni of Perugia, Italy, and described by him in a paper read before the Rome Medical Society on February 8. It consists essentially of an observation tube of opaque material, having its passage closed by a screen whose surface is coated with crystals of a substance which becomes fluorescent under the influence of X rays -such as platino-cyanide of barium. The eye is applied to the aperture at one end. The object to be examined is simply placed between the observation tube and the Crookes tube, and a shadow picture is thus at once produced on the fluorescent screen.

A similar device, constructed by E. P. Thompson of New York city, is called by him a "kinetoskotoscope," its purpose being to render instantly visible motions occurring within the interior of bodies. Mr. S. H. Emmens, whose theory of the universal emission of X rays from all sources of radiant energy has already been alluded to,

has constructed an instrument called a “photoscope," by means of which, he claims, it is possible to examine certain internal parts of the human body-as the bones-with the aid of sunlight only. Its principle is the same as in the cases already cited-the casting of a shadow upon a screen which fluoresces under the action of X rays.

But the most efficient of all the devices for the purpose indicated, is the "fluoroscope," invented by Thomas A. Edison. It consists of a pyramid-shaped tube-like the box of an old stereopticon-the smaller end of which is adjusted to the eyes so as to exclude all light. At the larger end is placed a piece of cardboard, on the inner side of which is pasted a screen of white cloth coated with fine crystals of tungstate of calcium,* which substance Mr. Edison found to be about eight times as sensitive as the ordinarily used platino-cyanide of barium. So readily does the screen thus prepared fluoresce under the action of X rays, that the bones of a human hand can be instantly seen in shadow pictures when held up even at a distance of fifteen feet in front of the Crookes tube; and Mr. Edison is reported to have gazed upon the bones of his own hand through eight inches of wood. The practical utility of the fluoroscope for the inspection of bone fractures, the location of bullets and other foreign substances imbedded in the organism, and other uses of surgery, is undoubtedly very great. In the laboratory it will also have the important function of giving to all experimenters with X rays an instant valuation of their tubes and of their excitation.

Up to the present time the practical applications of the discovery of X rays have been mainly in the fields of surgery and medical diagnosis. The dreams at first entertained-that by means of the new "photography" we should be enabled to watch the various functions of all the internal organs of the body, and detect at once all such morbid developments of tissue as tumors, cancers, etc.— seem to have been unwarranted. The radiotypes show no details except such as depend on varying thickness, and either density or atomic weight. It is thus probable that the X-ray pictures will serve merely to facilitate operations, rather than to effect a revolution in surgery or medical treatment. Their positive advantages, so far as at present known, seem to be limited to the determination of fractures, dislocations, malformations, and tumors of bones, and the location of encysted bullets, needles, pieces of glass, or other foreign bodies in the tissues, and earthy calculi. In cases of the inspection of fractures, dislocations, and other injuries calling for immediate treatment, the objectionable delays incident to the development of photographic prints of the injuries, are obviated by the fluoroscope.

*Tungstate of calcium is made by fusing together a mixture of sodium chloride, sodium tungstate, and calcium chloride. The calcium takes up the tungstic acid, sodium chloride being the other product of the double decomposition. Treatment with water dissolves out the sodium chloride, leaving the insoluble crystals of calcium tungstate, which are dried and sifted to the required fineness-about half the size of ordinary crystals of granulated sugar.

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