Most of us are able to recognise when we see them shadowgraphs taken by the aid of the now famous X-rays. They generally represent some part of the structure of men, beasts, birds, or fishes. Very dark patches show the position of the bones, large and small; lighter patches the more solid muscles clinging to the bony framework; and outside these again are shadowy tracts corresponding to the thinnest and most transparent portions of the fleshy envelope.

In an age fruitful as this in scientific marvels, it often takes some considerable time for the public to grasp the full importance of a fresh discovery. But when, in 1896, it was announced that Professor RÖntgen of WÜrzburg had actually taken photographs of the internal organs of still living creatures, and penetrated metal and other opaque substances with a new kind of ray, great interest was manifested throughout the civilised world. On the one hand the “new photography” seemed to upset popular ideas of opacity; on the other it savoured strongly of the black art, and, by its easy excursions through the human body, seemed likely to revolutionise medical and surgical methods. At first many strange ideas about the X-rays got afloat, attributing to them powers which would have surprised even their modest discoverer. It was also thought that the records were made in a camera after the ordinary manner of photography, but as a matter of fact RÖntgen used neither lens nor camera, the operation being similar to that of casting a shadow on a wall by means of a lamp. In X-radiography a specially constructed electrically-lit glass tube takes the place of the lamp, and for the wall is substituted a sensitised plate. The object to be radiographed is merely inserted between them, its various parts offering varying resistance to the rays, so that the plate is affected unequally, and after exposure may be developed and printed from it the usual way. Photographs obtained by using X-rays are therefore properly called shadowgraphs or skiagraphs.

The discovery that has made Professor RÖntgen famous is, like many great discoveries, based upon the labours of other men in the same field. Geissler, whose vacuum tubes are so well known for their striking colour effects, had already noticed that electric discharges sent through very much rarefied air or gases produced beautiful glows. Sir William Crookes, following the same line of research, and reducing with a Sprengel air-pump the internal pressure of the tubes to 1/100000 of an atmosphere, found that a luminous glow streamed from the cathode, or negative pole, in a straight line, heating and rendering phosphorescent anything that it met. Crookes regarded the glow as composed of “radiant matter,” and explained its existence as follows. The airy particles inside the tube, being few in number, are able to move about with far greater freedom than in the tightly packed atmosphere outside the tube. A particle, on reaching the cathode, is repelled violently by it in a straight line, to “bombard” another particle, the walls of the tube, or any object set up in its path, the sudden arrest of motion being converted into light and heat.

By means of special tubes he proved that the “radiant matter” could turn little vanes, and that the flow continued even when the terminals of the shocking-coil were outside the glass, thus meeting the contention of Puluj that the radiant matter was nothing more than small particles of platinum torn from the terminals. He also showed that, when intercepted, radiant matter cast a shadow, the intercepting object receiving the energy of the bombardment; but that when the obstruction was removed the hitherto sheltered part of the glass wall of the tube glowed with a brighter phosphorescence than the part which had become “tired” by prolonged bombardment. Experiments further revealed the fact that the shaft of “Cathode rays” could be deflected by a magnet from their course, and that they affected an ordinary photographic plate exposed to them.

In 1894 Lenard, a Hungarian, and pupil of the famous Hertz, fitted a Crookes’ tube with a “window” of aluminium in its side replacing a part of the glass, and saw that the course of the rays could be traced through the outside air. From this it was evident that something else than matter must be present in the shaft of energy sent from the negative terminal of the tube, as there was no direct communication between the interior and the exterior of the tube to account for the external phosphorescence. Whatever was the nature of the rays he succeeded in making them penetrate and impress themselves on a sensitised plate enclosed in a metal box.

Then in 1896 came RÖntgen’s great discovery that the rays from a Crookes’ tube, after traversing the glass, could pierce opaque matter. He covered the tube with thick cardboard, but found that it would still cast the shadows of books, cards, wood, metals, the human hand, &c., on to a photographic plate even at the distance of some feet. The rays would also pass through the wood, metal, or bones in course of time; but certain bodies, notably metals, offered a much greater resistance than others, such as wood, leather, and paper. Professor RÖntgen crowned his efforts by showing that a skeleton could be “shadow-graphed” while its owner was still alive.

Naturally everybody wished to know not only what the rays could do, but what they were. RÖntgen, not being able to identify them with any known rays, took refuge in the algebraical symbol of the unknown quantity and dubbed them X-rays. He discovered this much, however, that they were invisible to the eye under ordinary conditions; that they travelled in straight lines only, passing through a prism, water, or other refracting bodies without turning aside from their path; and that a magnet exerted no power over them. This last fact was sufficient of itself to prevent their confusion with the radiant matter “cathode rays” of the tube. RÖntgen thought, nevertheless, that they might be the cathode rays transmuted in some manner by their passage through the glass, so as to resemble in their motion sound-waves, i.e. moving straight forward and not swaying from side to side in a series of zig-zags. The existence of such ether waves had for some time before been suspected by Lord Kelvin.

Other authorities have other theories. We may mention the view that X represents the ultra-violet rays of the spectrum, caused by vibrations of such extreme rapidity as to be imperceptible to the human eye, just as sounds of extremely high pitch are inaudible to the ear. This theory is to a certain extent upheld by the behaviour of the photographic plate, which is least affected by the colours of the spectrum at the red end and most by those at the violet end. A photographer is able to use red or orange light in his dark room because his plates cannot “see” them, though he can; whereas the reverse would be the case with X-rays. This ultra-violet theory claims for X-rays a rate of ether vibration of trillions of waves per second.

An alternative theory is to relegate the rays to the gap in the scale of ether-waves between heatwaves and light-waves. But this does not explain any more satisfactorily than the other the peculiar phenomenon of non-refraction.

The apparatus employed in X-photography consists of a Crookes’ tube of a special type, a powerful shocking or induction coil, a fluorescent screen and photographic plates and appliances for developing, &c., besides a supply of high-pressure electricity derived from the main, a small dynamo or batteries.

A Crookes’ tube is four to five inches in diameter, globular in its middle portion, but tapering away towards each end. Through one extremity is led a platinum wire, terminating in a saucer-shaped platinum plate an inch or so across. At the focus of this, the negative terminal, is fixed a platinum plate at an angle to the path of the rays so as to deflect them through the side of the tube. The positive terminal penetrates the glass at one side. The tube contains, as we have seen, a very tiny residue of air. If this were entirely exhausted the action of the tube would cease; so that some tubes are so arranged that when rarefaction becomes too high the passage of an electrical current through small bars of chemicals, whose ends project through the sides of the tube, liberates gas from the bars in sufficient quantity to render the tube active again.

When the Ruhmkorff induction coil is joined to the electric circuit a series of violent discharges of great rapidity occur between the tube terminals, resembling in their power the discharge of a Leyden jar, though for want of a dense atmosphere the brilliant spark has been replaced by a glow and brush-light in the tube. The coil is of large dimensions, capable of passing a spark across an air-gap of ten to twelve inches. It will perhaps increase the reader’s respect for X-rays to learn that a coil of proper size contains upwards of thirteen miles of wire; though indeed this quantity is nothing in comparison with the 150 miles wound on the huge inductorium formerly exhibited at the London Polytechnic.

If we were invited to an X-ray demonstration we should find the operator and his apparatus in a darkened room. He turns on the current and the darkness is broken by a velvety glow surrounding the negative terminal, which gradually extends until the whole tube becomes clothed in a green phosphorescence. A sharply-defined line athwart the tube separates the shadowed part behind the receiving plate at the negative focus—now intensely hot—from that on which the reflected rays fall directly.

One of us is now invited to extend a hand close to the tube. The operator then holds on the near side of the hand his fluorescent screen, which is nothing more than a framework supporting a paper smeared on one side with platino-cyanide of barium, a chemical that, in common with several others, was discovered by Salvioni of Perugia to be sensitive to the rays and able to make them visible to the human eye. The value of the screen to the X-radiographer is that of the ground-glass plate to the ordinary photographer, as it allows him to see exactly what things are before the sensitised plate is brought into position, and in fact largely obviates the necessity for making a permanent record.

The screen shows clearly and in full detail all the bones of the hand—so clearly that one is almost irresistibly drawn to peep behind to see if a real hand is there. One of us now extends an arm and the screen shows us the ulna and the radius working round each other, now both visible, now one obscuring the other. On presenting the body to the course of the rays a remarkable shadow is cast on to the screen. The spinal column and the ribs; the action of the heart and lungs are seen quite distinctly. A deep breath causes the movement of a dark mass—the liver. There is no privacy in presence of the rays. The enlarged heart, the diseased lung, the ulcerated liver betrays itself at once. In a second of time the phosphorescent screen reveals what might baulk medical examination for months.

If a photographic slide containing a dry-plate be substituted for the focusing-screen, the rays soon penetrate any covering in which the plate may be wrapped to protect it from ordinary light rays. The process of taking a shadowgraph may therefore be conducted in broad daylight, which is under certain conditions a great advantage, though the sensitiveness of plates exposed to RÖntgen rays entails special care being taken of them when they are not in use. In the early days of X-radiography an exposure of some minutes was necessary to secure a negative, but now, thanks to the improvements in the tubes, a few seconds is often sufficient.

The discovery of the X-rays is a great discovery, because it has done much to promote the noblest possible cause, the alleviation of human suffering. Not everybody will appreciate a more rapid mode of telegraphy, or a new method of spinning yarn, but the dullest intellect will give due credit to a scientific process that helps to save life and limb. Who among us is not liable to break an arm or leg, or suffer from internal injuries invisible to the eye? Who among us therefore should not be thankful on reflecting that, in event of such a mishap, the X-rays will be at hand to show just what the trouble is, how to deal with it, and how far the healing advances day by day? The X-ray apparatus is now as necessary for the proper equipment of a hospital as a camera for that of a photographic studio.

It is especially welcome in the hospitals which accompany an army into the field. Since May 1896 many a wounded soldier has had reason to bless the patient work that led to the discovery at WÜrzburg. The Greek war, the war in Cuba, the Tirah campaign, the Egyptian campaign, and the war in South Africa, have given a quick succession of fine opportunities for putting the new photography to the test. There is now small excuse for the useless and agonising probings that once added to the dangers and horrors of the military hospital. Even if the X-ray equipment, by reason of its weight, cannot conveniently be kept at the front of a rapidly moving army, it can be set up in the “advanced” or “base” hospitals, whither the wounded are sent after a first rough dressing of their injuries. The medical staff there subject their patients to the searching rays, are able to record the exact position of a bullet or shell-fragment, and the damage it has done; and by promptly removing the intruder to greatly lessen its power to harm.

The RÖntgen ray has added to the surgeon’s armoury a powerful weapon. Its possibilities are not yet fully known, but there can be no doubt that it marks a new epoch in surgical work. And for this reason Professor RÖntgen deserves to rank with Harvey, the discoverer of the blood’s circulation; with Jenner, the father of vaccination; and with Sir James Young Simpson, the first doctor to use chloroform as an anÆsthetic.

Photography in the Dark.

Strange as it seems to take photographs with invisible rays, it is still stranger to be able to affect sensitised plates without apparently the presence of any kind of rays.

Professor W. J. Russell, Vice-President of the Royal Society of London, has discovered that many substances have the power of impressing their outlines automatically on a sensitive film, if the substance be placed in a dark cupboard in contact with, or very close to a dry-plate.

After some hours, or it may be days, development of the plate will reveal a distinct impression of the body in question. Dr. Russell experimented with wood, metal, leaves, drawings, printed matter, lace. Zinc proved to be an unusually active agent. A plate of the metal, highly polished and then ruled with patterns, had at the end of a few days imparted a record of every scratch and mark to the plate. And not only will zinc impress itself, but it affects substances which are not themselves active, throwing shadowgraphs on to the plate. This was demonstrated with samples of lace, laid between a plate and a small sheet of bright zinc; also with a skeleton leaf. It is curious that while the interposition of thin films of celluloid, gutta-percha, vegetable parchment, and gold-beater’s skin—all inactive—between the zinc and the plate has no obstructive effect, a plate of thin glass counteracts the action of the zinc. Besides zinc, nickel, aluminium, pewter, lead, and tin among the metals influence a sensitised plate. Another totally different substance, printer’s ink, has a similar power; or at least some printer’s ink, for Professor Russell found that different samples varied greatly in their effects. What is especially curious, the printed matter on both sides of a piece of newspaper appeared on the plate, and that the effect proceeded from the ink and not from any rays passing from beyond it is proved by the fact that the type came out dark in the development, whereas if it had been a case of shadowgraphy, the ink by intercepting rays would have produced white letters. Professor Russell has also shown that modern writing ink is incapable of producing an impression unaided, but that on the other hand paper written on a hundred years ago or a printed book centuries old will, with the help of zinc, yield a picture in which even faded and uncertain characters appear quite distinctly. This opens the way to a practical use of the discovery, in the deciphering of old and partly obliterated manuscripts.

A very interesting experiment may be made with that useful possession—a five-pound note. Place the note printed side next to the plate, and the printing appears dark; but insert the note between a zinc sheet and the plate, its back being this time towards the sensitised surface, and the printing appears white; and the zinc, after contact with the printed side, will itself yield a picture of the inscription as though it had absorbed some virtue from the note!

As explanation of this paradoxical dark photography—or whatever it is—two theories may be advanced. The one—favoured by Professor Russell—is that all “active” substances give off vapours able to act on a photographic plate. In support of this may be urged the fact that the interposition of glass prevents the making of dark pictures. But on the other hand it must be remembered that celluloid and sheet-gelatine, also air-tight substances, are able to store up light and to give it out again. It is well known among photographers that to allow sunlight to fall on the inside of a camera is apt to have a “fogging” effect on a plate that is exposed in the camera afterwards, though the greatest care be taken to keep all external light from the plate. But here the glass again presents a difficulty, for if this were a case of reflected light, glass would evidently be less obstructive than opaque vegetable parchment or gutta-percha.