As to the nature of the Röntgen rays, Professor Wright is inclined to regard them as a mode of motion through the ether, in longitudinal stresses; and he thinks that, while they are in many ways similar to the rays discovered by Lenard a year or so ago, they still present important characteristics of their own. It may be, he thinks, that the Röntgen rays are the ordinary cathode rays produced in a Crookes tube, filtered, if one may so express it, of the metallic particles carried in their electrical stream from the metal terminal, on passing through the glass. It is well known that the metal terminals of a Crookes tube are steadily worn away while the current is passing; so much so that sometimes portions of the interior of the tube become coated with a metallic deposit almost mirror-like.
As to the future, Professor Wright feels convinced that important results will be achieved in surgery and medicine by the use of these new rays, while in physical science they point to an entirely new field of investigation. The most necessary thing now is to find some means of producing streams of Röntgen rays of greater volume and intensity, so as to make possible greater penetration and distinctness in the images. Thus far only small Crookes tubes have been used, and much is to be expected when larger ones become available; but there is great difficulty in the manufacture of them. It might be possible, Professor Wright thinks, to get good results by using, instead of the Crookes tube, a large sphere of aluminium, which is more transparent to the new rays than glass and possesses considerable strength. It is a delicate question, however, whether the increased thickness of metal necessary to resist the air pressure upon a vacuum would not offset the advantage gained from the greater size. Moreover, it is a matter for experiment still to determine, what kind of an electric current would be necessary to excite such a larger tube with the best results.
Among the most important experiments in shadow photography made thus far in America are those of Dr. William J. Morton of New York, who was the first in this country to use the disruptive discharges of static electricity in connection with the Röntgen discovery, and to demonstrate that shadow pictures may be successfully taken without the use of Crookes tubes. It was the well-known photographic properties of ordinary lightning that made Dr. Morton suspect that cathode rays are produced freely in the air when there is an electric discharge from the heavens. Reasoning thus, he resolved to search for cathode rays in the ten-inch lightning flash he was able to produce between the poles of his immense Holtz machine, probably the largest in this country.
On January 30th he suspended a glass plate, with a circular window in the middle, between the two poles. Cemented to this plate of glass was one of hard rubber, about equal in size, which of course covered the window in the glass. Back of the rubber plate was suspended a photographic plate in the plate-holder, and outside of this, between it and the rubber surface, were ten letters cut from thin copper. Dr. Morton proposed to see if he could not prove the existence of cathode rays between the poles by causing them to picture in shadow, upon the sensitized plate, the letters thus exposed.
In order to do this it was necessary to separate the ordinary electric sparks from the invisible cathode rays which, as Dr. Morton believed, accompanied them. It was to accomplish this that he used the double plates of glass and hard rubber placed, as already described, between the two poles; for while the ordinary electric spark would not traverse the rubber, any cathode rays that might be present would do so with great ease, the circular window in the glass plate allowing them passage there.
DR. WILLIAM J. MORTON PHOTOGRAPHING HIS OWN HAND UNDER RÖNTGEN RAYS.
In this case the vacuum bulb is charged from Leyden jars which, in their turn, are excited by an induction coil.
The current being turned on, it was found that the powerful electric sparks visible to the eye, unable to follow a straight course on account of the intervening rubber plate, jumped around the two plates in jagged, lightning-like lines, and thus reached the other pole of the machine. But it was noticed that at the same time a faint spray of purplish light was streaming straight through the rubber between the two holes, as if its passage was not interfered with by the rubber plate. It was in company with this stream of violet rays, known as the brush discharge, that the doctor conceived the invisible Röntgen rays to be projected at each spark discharge around the plate; and presently, when the photographic plate was developed, it was found that his conception was based on fact. For there, dim in outline, but unmistakable, were shadow pictures of the ten letters which stand as historic, since they were probably the first shadow pictures in the world taken without any bulb or vacuum tube whatever. These shadow pictures Dr. Morton carefully distinguished from the ordinary blackening effects on the film produced by electrified objects.
Pursuing his experiments with static electricity, Dr. Morton soon found that better results could be obtained by the use of Leyden jars influenced by the Holtz machine, and discharging into a vacuum bulb, as shown in the illustration on this page. This arrangement of the apparatus has the advantage of making it much easier to regulate the electric supply and to modify its intensity, and Dr. Morton finds that in this way large vacuum tubes, perhaps twenty inches in diameter, may be excited to the point of doing practical work without danger of breaking the glass walls. But certain precautions are necessary. When he uses tin-foil electrodes on the outside of the bulb, he protects the tin-foil edges, and, what is more essential, uses extremely small Leyden jars and a short spark gap between the poles of the discharging rods. The philosophy of this is, that the smaller the jars, the greater their number of oscillations per second (easily fifteen million, according to Dr. Lodge's computations), the shorter the wave length, and, therefore, the greater the intensity of effects.