Popular Science Monthly/Volume 50/March 1897/The Year of the X Rays
By D. W. HERING,
PROFESSOR OF PHYSICS, NEW YORK UNIVERSITY.
THE incredulity which greeted the first reports of Prof. Röntgen's famous discovery gave place, upon their confirmation, to a delirium of enthusiasm, experimentation, and expectation. So startling and so novel were the facts reported by the discoverer that no prediction seemed too wild, no penetration into the unknown either impossible or improbable. The condition of mind actually prevailing at that time with a large number of persons is admirably shown by President Jordan's amusing article on the Sympsychograph in the Popular Science Monthly for September, 1896.
Prof. Röntgen reported his investigations in a paper before the Physico-Medical Society of Würzburg, in December, 1895. The account of his paper was transmitted to America in a few brief statements, January 7, 1896, the full report not arriving until some weeks later. Popular interest was focused upon the fact that the X rays, as its discoverer provisionally named the mysterious agency, would reveal a bony skeleton within its case of fleshy tissue, and the famous picture of a hand in which the bones thus stood revealed was soon to be found in every city of Europe and America. The realism of this weird picture simply fascinated all who beheld it. Attempts were made to repeat and extend the original experiments wherever there was any semblance of apparatus suited to the purpose. An electric potential high enough to give a spark of several inches in air, and a vacuum tube in which the spark was to be discharged, seemed to be requisites, and wherever these were obtainable the experiments were attempted. Poor facilities led to efforts to dispense with good ones, the prevailing meagerness of equipment becoming thereby a means all the earlier of deciding the limitations for successful operation. Of course, the excitement quieted when the novelty wore off, but investigations in this new field must continue for a long time.
The pure physics of the subject was, naturally, the side which most appealed to scientific professors. How was the strange agent to be set to work, and how did it work? Was it light, or was it electricity? Was it material, or ethereal? Was it due to the cathode or to the anode terminal of the vacuum tube? The year's work upon these questions leaves them answered only partially and unsatisfactorily. Very little indeed has been added to the facts brought out by Dr. Röntgen in the first instance. The Physical Society of London, in its abstracts of physical papers from foreign sources, classes all work with the Röntgen rays under the head of "Light," but upon very scant grounds. Numerous experiments have been made to test the character of these mysterious activities by the accepted criteria of light—namely, reflection, refraction, interference, and polarization—all results being negative, or so slight and uncertain as to leave them still open to question, and to make the name "X rays" not only the most common one by which they are mentioned, but the one best suited to express our knowledge—or ignorance—of their nature. Several attempts have been made to determine a length for them, supposing them to be waves, resulting in a supposed upper limit of length not greater than one hundredth that of violet light, and probably not greater than one three-hundredth. In the first few months of the furore of experimentation and discussion scarcely a result was announced by one observer that was not controverted by another; yet out of this very contradictoriness came a rational conclusion that at all events the rays are not homogeneous, but differ among themselves in their properties, as do the constituent rays of ordinary heterogeneous light. This would account for their noninterference. Of refraction there is as yet no evidence, nor, so far as known, is there any possibility of bringing the rays to a focus and thus producing an image of any object by means of them. All that can be done in that way as yet, as at first, is to obtain a shadow of varying intensity by reason of the various penetrability of different objects or portions of one object; and so the pictures thus produced are called by various names, as skiagraphs, radiographs, X-ray pictures, etc.—all chosen to avoid the idea that they are real light-pictures or photographs. Since these shadows are produced by straight rays from a small surface, they are usually as large as the object itself, or larger. Many experiments were made to determine the source from which the rays proceed before it was learned definitely that they emanate from the surface upon which the cathode rays first impinge a fact that was announced almost simultaneously by several experimenters. It is one of the important points that have been determined, and even this was distinctly intimated by Prof. Röntgen in the twelfth section of his original paper.
In intensity they vary inversely as the square of the distance from their source.
They electrify some bodies positively and some negatively, and whatever charge a body may already have they reduce or change it to the charge which they would independently give to the body. Their penetrating power depends upon the length of time they act.
Thus, gradually, these and many additional isolated facts have been established, and no doubt enough data will be accumulated eventually to permit generalization into laws; but that stage has not yet been reached.
Four theories have been suggested:
1. "They are ether waves, like ordinary light, but of exceedingly brief period, therefore ultra ultra-violet."
2. "They are streams of material particles."
3. "They are vortices of the intermolecular ether, forced from the cathode when the gas pressure is sufficiently low. Rectilinear propagation, absence of reflection, etc., follow from the properties of vortices."
4. "They are variations of stress in the dielectric surrounding the vacuum tubes."
Each of these theories is entitled to the Scotch verdict "Not proven," though the preponderance of opinion is on the side of the first. Still, it can not yet be said to be more than opinion.
Of the hundreds of papers that have been written during the year, the greater number have had reference to some special feature of manipulation, or detail of action of the rays, so that more has been learned of how to work with them than of their essential character. This has led naturally to improved apparatus.
It is well to keep in mind that the X rays do not make objects visible by their direct action, as light does. They do make certain substances self-luminous, causing them to emit a soft light of a grayish-blue or yellow or green color, depending on the nature of the substance, but this color is ordinary light, and not, at least to any considerable extent, the X rays. This luminosity, called fluorescence, is also excited in many substances by the ultra-violet or colorless portion of light, but the X rays are especially strong in producing it. The substance employed by Dr. Röntgen was barium platino-cyanide, which is expensive. Experiment soon showed that other substances were more efficient as well as cheaper, hundreds having been tested under the rays for this effect. The best are tungstate of calcium, tungstate of zinc, barium platino-cyanide, and potassium platino-cyanide, the first named being at present the most common, though the last named has been a favorite with English experimenters. A screen of cardboard covered with a layer of fine crystals of either of these substances and exposed to the rays in a dark room, immediately lights up under their action, and a body impervious to the rays, when placed before the screen, is seen upon it as a shadow. If this screen is the front end of a light-proof box, into the other end of which the eyes can look while all light is excluded, we then have the fluoroscope, by which examinations can be made in a lighted room. Probably few X rays pass through and beyond the fluoroscopic screen. The effect of radiant energy upon a body is determined not by the rays that pass through it, but by those that are absorbed by it. It is difficult, therefore, to understand how the light which the blind have been said to see on peering into a fluoroscope can be really due to X rays.
The invention and improvement of the fluoroscope constitute an important part of the progress that has been made. The effect of the rays on photographic plates is heightened by similar means. The sensitive plate to be exposed to the rays is itself carefully in-closed in a wrapper so as to shut out every trace of light. If, before thus wrapping up the plate, a fluoroscopic screen is placed with its surface of crystals directly in contact with the photographic film, then where the rays penetrate to this crystalline surface it becomes luminous, and the light immediately affects the sensitive plate except in those spots where the object intercepts the X rays, and where consequently they do not cause fluorescence of the screen. This device has greatly reduced the time needed to obtain a photographic impression. Fig. 1 is an illustration of such action. A photographic plate was partly covered by such a screen, and the hand was placed partly over the screen and partly over the plate not covered by the screen. An exposure of twelve seconds was more than sufficient to produce a strong picture of the interior of the hand, under the screen, the flesh almost disappearing from view, while the effect upon the other portion of the plate (the dark part) is much less pronounced. The line of demarcation is very sharp.Also photographic plates or films have been adapted to this particular use by preparing them so as to absorb the energy of the rays. X-ray plates are now used of which the mode of preparation is of course the manufacturer's secret, but which are
Fig. 1.—Effect of Fluoroscopic Screen. Exposure, twelve seconds.
coated with a very thick film apparently impregnated with some substance that fluoresces under the X rays. The time of exposure of such plates is less than with ordinary ones, though not much less than is required for a quick plate covered by the fluorescent screen, but the latter will not give the detail and differentiation of parts which are unequally penetrable by the rays that can be got from the X-ray plates.
The rays also affect sensitive paper, especially bromide paper, and now so-called X-ray paper is in use requiring even briefer exposures than plates. The picture on such paper is a negative—that is, shadows are light and parts affected by the rays are dark. Fig. 2 is an example of a picture taken on such paper, the objects being such as were greatly in vogue for the early pictures—a purse, a pincushion, etc. In the early efforts such a picture required
Fig. 2.—An Early Picture Repeated. Exposure, two and a half seconds.
fully twenty minutes' exposure to the rays; the example here was produced in two and a half seconds, or about one five-hundredth part of the former time. The writer has obtained a perfectly distinct picture of the same kind by a single fluorescent flash in the tube. That is practically instantaneous.
Fig. 3 shows the principal changes in style of tubes that have been approved. Nos. (1), (2), and (3) are forms that were to be found in most collections of Crookes's tubes in physical laboratories when the X rays were first made known. No. (1) was one of the earliest to give satisfactory results; then (2) was found to be preferable, and this "pear shape" was recommended as the most suitable form. Almost at the same time No. (3) was found to be particularly efficient. In this the cathode rays converge from a concave terminal upon a platinum plate used as an anode, such plate becoming the source of Röntgen rays proper. This form was immediately developed into what was called a "focus tube" in which a similar concave focusing cathode was employed, and a platinum plate inclined at forty-five degrees to the axis of the cathode rays was inserted between the two electrodes of the tube to receive the impact of the cathode rays as in No. (4). A plate so placed is called an anticathode. This idea was carried still further to produce the double-focus tube shown in No. (5), which is especially suited to oscillatory discharges from the electrodes, and therefore adapted to use with alternating current apparatus, especially Tesla coils. In this form of tube the anticathode
consists of a wedge-shaped piece of platinum midway between the ends of the tube. If this platinum terminal be connected with the positive pole and both end electrodes with the negative pole, this tube is very efficient with a Ruhmkorff coil giving unidirectional discharges. For any tube there is a critical degree of vacuum as well as electric potential, with which it is most efficient. Tubes can be made suitable for a coil giving a spark of not more than an inch, but they are not very energetic. Since the vacuum rises with continued use of a tube, some forms—e. g.. No, (5)—have a small side tube communicating with the main bulb and containing caustic potash or other substance which volatilizes on being heated, so that its vapor will reduce the vacuum. These are called adjustable vacuum tubes, and afford a means of controlling the requisite sparking gap of the coil within certain limits. Nos. (4) and (5) are now almost the only styles of tubes that meet with favor.
Three types of apparatus have been employed in exciting the X rays. All are necessarily such as are capable of producing a high electric potential, and all were in use prior to Dr. Röntgen's discovery. They are the Ruhmkorff induction coil, the plate influence machine (either the Wimshurst or the Töpler-Holtz), and the Tesla coil. The only development in these machines has been in some instances the improvement of their quality and enlargement of their capacity—without, however, introducing any novelty in the type of the apparatus, unless we except making the condenser of the induction coil adjustable in capacity. The most suitable rate of interruption of the primary current for each coil and tube may best be found by trial. Where a continuous current is supplied from a commercial circuit of a hundred and ten volts or more a rotating segmental wheel as interrupter with a rheostat in circuit is of advantage, but many experimenters get as good results by using a storage battery of six to ten cells, with an ordinary hammer break in the coil. The Ruhmkorff coil is used to give a unidirectional discharge in the Crookes's tube. Influence machines having several rotating plates act in the same way, and with excellent effect. Tesla coils are employed to give exceedingly rapid discharges to and fro in the tube, which require, therefore, two terminals that can both act as cathodes. It can not be said that either of these three forms is per se the best. With proper accessories one will give as good results as another, but the ordinary induction coil with suitable single-focus tube is the most generally practicable.
Fig. 4 shows an outfit of apparatus for X-ray use. It consists, in this instance, of a variable rheostat connected to one main of a hundred-and-ten-volt continuous current; in series with this is, next, a rotary interrupter, which is also driven by a current from the same main circuit; then comes an ammeter, then a pole changer or reverser, which connects back to the other main and forward to the primary of the large Ruhmkorff coil. This coil has in its base a condenser which is united with an additional adjustable condenser. There are, further, a double-focus tube, fluoroscope, and screen.
The most obvious suggestion of usefulness for the new agent was in surgery. It was so easy to discover any foreign substance in portions of the body, or to perceive the nature of any bony malformation, that it was hoped that surgery had received a valuable assistant in these rays. From time to time reports of successful operations based upon such revelations have been made, but the early expectations were exaggerated. Methods of making examinations by these means have been so far simplified as to require no highly specialized knowledge for this purpose, and one would expect that hospitals, at all events, would be provided with an X-ray outfit if there is any advantage in it. Replies from a large number of prominent hospitals in six of the leading cities of America, which were asked concerning their employment of the X rays, showed that, of those replying, one third have such outfits; about one fifth have none, but expect to have one soon; and nearly half of those without such equipment have had examinations made for them. All that have used the rays testify to
Fig, 4.—An X-Ray Outfit.
their helpfulness, some of the physicians being enthusiastic over the method. Enough is told to show that the X ray is already an important aid to diagnosis, and, unless the future experience of the hospitals should be quite disappointing, such apparatus will soon be thought an indispensable feature of their equipment. The interior of the trunk, as well as of the limbs, has been successfully shown, the fluoroscopic revelation being immediate, while for photographic reproduction exposures of varying lengths of time are needed. The hand is the easiest member, requiring from five to thirty seconds, while the trunk requires half an hour or more. In general, it may be said that for pictures showing distinctions of structure, the time now required is from one hundredth to one fiftieth of that necessary at first. Pictures thus taken are being supplied to schools for the use of classes in anatomy and physiology.