On the evening of Friday 8 November 1895, the Professor of Physics at the University of Würzburg, Wilhelm Conrad Roentgen (1845–1923) was working late in his laboratory, after everyone else had gone home. He was preparing to carry out some experiments with an induction coil connected to the electrodes of a partially evacuated glass tube.

It had been known since 1858 that when electricity was discharged through the air or other gases in such tubes, the glass became phosphorescent. The ‘cathode rays’ causing this had been fancifully described by Sir William Crookes in 1878 as ‘a stream of molecules in flight’. It was also known that if the tube had a thin metal-foil ‘window’ in it, the cathode rays would penetrate this and cause fluorescence a few centimetres beyond the tube. Roentgen thought cathode rays might be detectable outside the tube even when there was no metal-foil window. As a first step in investigating this he covered the entire tube in black cardboard, and drew the curtains to darken the room. To test that his cardboard shield would not let light through, he then turned on the high-voltage coil and passed a current through the tube.

What happened next is described by Roentgen’s student Charles Nootnangle of Minneapolis, who had it from Roentgen himself a few days later:

By chance he happened to note that a little piece of paper lying on his work table was sparkling as though a single ray of bright sunshine had fallen upon it lying in the darkness. At first he thought it was merely the reflection from the electric spark, but the reflection was too bright to allow that explanation. Finally he picked up the piece of paper and, examining it, found that the reflected light was given by a letter A which had been written on the paper with a platinocyanide [fluorescent] solution.

It was at once clear to Roentgen that the piece of chemically-treated paper could not have been made to fluoresce by cathode rays, since it was several feet away from the tube. Some other rays must be responsible – rays that were able to pass through the cardboard shield round the tube, and travel invisibly through the air. Since he had no idea what these rays were, Roentgen called them X-rays, and he began experimenting to see what other substances they would pass through. On 28 December 1895 he presented his ‘Preliminary Communication’, entitled On a new Kind of Rays, to the President of the Würzburg Physical and Medical Society.

The most striking feature of this phenomenon is the fact that an active agent here passes through a black cardboard envelope, which is opaque to the visible and the ultra-violet rays of the sun or of the electric arc; an agent, too, which has the power of producing active fluorescence. Hence we may first investigate the question whether other bodies also possess this property.

We soon discover that all bodies are transparent to this agent, though in very different degrees. I proceed to give a few examples: Paper is very transparent; behind a bound book of about one thousand pages I saw the fluorescent screen light up brightly, the printers’ ink offering scarcely a notable hindrance. In the same way the fluorescence appeared behind a double pack of cards; a single card held between the apparatus and the screen being almost unnoticeable to the eye. A single sheet of tin-foil is also scarcely perceptible; it is only after several layers have been placed over one another that their shadow is distinctly seen on the screen. Thick blocks of wood are also transparent, pine boards two or three centimetres thick absorbing only slightly. A plate of aluminium about fifteen millimetres thick, though it enfeebled the action seriously, did not cause the fluorescence to disappear entirely. Sheets of hard rubber several centimetres thick still permit the rays to pass through them. (For brevity’s sake I shall use the expression ‘rays’; and to distinguish them from others of this name I shall call them ‘X-rays’.) Glass plates of equal thickness behave quite differently, according as they contain lead (flint-glass) or not; the former are much less transparent than the latter. If the hand be held between the discharge-tube and the screen, the darker shadow of the bones is seen within the slightly dark shadow-image of the hand itself … Lead of a thickness of 1.5 millimetres is practically opaque …

I have observed, and in part photographed, many shadow-pictures of this kind, the production of which has a particular charm. I possess, for instance, photographs of the shadow of the profile of a door which separates the rooms in which, on one side, the discharge-apparatus was placed, on the other the photographic plate; the shadow of the bones of the hand; the shadow of a covered wire wrapped on a wooden spool; of a set of weights enclosed in a box; of a galvanometer in which the magnetic needle is entirely enclosed by metal; of a piece of metal whose lack of homogeneity becomes noticeable by means of the X-rays, etc. I have obtained a most beautiful photographic shadow-picture of the double barrels of a hunting-rifle with cartridges in place, in which all the details of the cartridges, the internal faults of the damask barrels, etc., could be seen most distinctly and sharply.

In addition to his own language he speaks French well and English scientifically, which is different from speaking it popularly. These three tongues being more or less within the equipment of his visitor, the conversation proceeded on an international or polyglot basis, so to speak, varying at necessity’s demand.

‘Now then,’ he said smiling and with some impatience, when some personal questions at which he chafed were over, ‘you have come to see the invisible rays.’

‘Is the invisible visible?’

‘Not to the eye, but its results are. Come in here.’

He led the way to a square room and indicated the induction coil with which his researches were made, an ordinary Ruhmkorff coil with a spark of from 4 to 6 in., charged by a current of twenty amperes. Two wires led from the coil through an open door into a smaller room on the right. In this room was a small table carrying a Crookes’ tube connected with the coil. The most striking object in the room, however, was a huge and mysterious tin [actually zinc and lead] box about 7 ft. high and 4 ft. square. It stood on end like a huge packing case, its side being perhaps 5 in. from the Crookes’ tube.

The professor explained the mystery of the tin box, to the effect that it was a device of his own for obtaining a portable dark room. When he began his investigations he used the whole room as was shown by the heavy blinds and curtains so arranged as to exclude the entrance of all interfering light from the windows. In the side of the tin box at the point immediately against the tube was a circular sheet of aluminium 1 mm. in thickness, and perhaps 18 in. diameter, soldered to the surrounding tin. To study his rays the professor had only to turn on the current, enter the box, close the door, and in perfect darkness inspect only such light or light effects as he had a right to consider his own, hiding his light, in fact, not under the Biblical bushel but in a more commodious box.

‘Step inside,’ said he, opening the door which was on the side of the box farthest from the tube. I immediately did so, not altogether certain whether my skeleton was to be photographed for general inspection or my secret thoughts held up to light on a glass plate. ‘You will find a sheet of barium paper on the shelf,’ he added, and then went away to the coil. The door was closed and the interior of the box became black darkness. The first thing I found was a wooden stool on which I resolved to sit. Then I found the shelf on the side next the tube, and then the sheet of paper prepared with barium platinocyanide. I was thus being shown the first phenomenon which attracted the discoverer’s attention and led to the discovery, namely, the passage of rays, themselves wholly invisible, whose presence was only indicated by the effect they produced on a piece of sensitized photographic paper.

A moment later, the black darkness was penetrated by the rapid snapping sound of the high-pressure current in action, and I knew that the tube outside was glowing. I held the sheet vertically on the shelf, perhaps 4 in. from the plate. There was no change, however, and nothing was visible.

‘Do you see anything?’

‘No.’

‘The tension is not high enough,’ and he proceeded to increase the pressure by operating an apparatus of mercury in long vertical tubes acted upon automatically by a weight lever which stood near the coil. In a few moments the sound of the discharge again began, and then I made my first acquaintance with the roentgen rays.

The moment the current passed, the paper began to glow. A yellowish-green light spread all over its surface in clouds, waves, and flashes. The yellow-green luminescence, all the stranger and stronger in the darkness, trembled, wavered, and floated over the paper, in rhythm with the snapping of the discharge. Through the metal plate, the paper, myself, and the tin box, the visible rays were flying, with an effect strange, interesting, and uncanny. The metal plate seemed to offer no appreciable resistance to the flying force, and the light was as rich and full as if nothing lay between the paper and the tube.

‘Put the book up,’ said the professor.

I felt upon the shelf, in the darkness, a heavy book, 2 in. in thickness, and placed this against the plate. It made no difference. The rays flew through the metal and the book as if neither had been there, and the waves of light, rolling cloud-like over the paper, showed no change in brightness. It was a clear, material illustration of the ease with which paper and wood are penetrated. And then I laid the book and paper down, and put my eyes against the rays. All was blackness, and I neither saw nor felt anything. The discharge was in full force, and the rays were flying through my head, and, for all I knew, through the side of the box behind me. But they were invisible and impalpable. They gave no sensation whatever. Whatever the mysterious rays may be, they are not to be seen and are to be judged only by their works.

I was loath to leave this historical tin box, but the time pressed. I thanked the professor, who was happy in the reality of his discovery, and the music of his sparks. Then I said, ‘Where did you first photograph living bones?’

‘Here,’ he said, leading the way into the room where the coil stood. He pointed to a table on which was another – the latter a small, short-legged wooden one, with more the shape and size of a wooden seat. It was 2 ft. square and painted coal black.

‘How did you take the first hand photograph?’

The professor went over to a shelf by the window, where lay a number of prepared glass plates, closely wrapped in black paper. He put a Crookes’ tube underneath the table, a few inches from the under side of its top. Then he laid his hand flat on the top of the table, and placed the glass plate loosely on his hand.

‘You ought to have your portrait painted in that attitude,’ I suggested.

‘No, that is nonsense,’ he said, smiling.

‘Or be photographed.’ This suggestion was made with a deeply hidden purpose.

The rays from the Röntgen eyes instantly penetrated the deeply hidden purpose. ‘Oh, no,’ said he, ‘I can’t let you make pictures of me. I am too busy.’ Clearly the professor was entirely too modest to gratify the wishes of the curious world.

The reception of the discovery by the public was not entirely favourable. Photographing the skeleton of a living person was felt to be eerie. A Professor Czermak of Graz was so appalled to see an X-ray photograph of his skull that he could not sleep. ‘He has not closed an eye since he saw his own death’s head,’ reported the Grazer Tageblatt. The possibility of seeing other people’s internal organs was widely considered a threat to privacy. But enthusiasm outweighed disapproval, and many potential uses of the new technique were suggested. In Paris a Dr Baraduc claimed that he could photograph the human soul with X-rays, and presented 400 such plates at an exhibition in Munich. During 1896 the use of X-rays in medical diagnosis was rapidly explored worldwide, especially in the USA. Photographs of a human foetus, of a tubercular patient’s lungs, and of the stomach, heart and other organs were published, and a Harvard professor, W. B. Cannon, watched pearl buttons pass down the oesophagus of a dog. The harmful effects of exposure to X-rays were soon noticed. Many cases of severe skin burns and loss of hair were reported, but no one appreciated the real danger. Noting their depilatory effect one enterprising Frenchman, M. Gaudoin of Dijon, offered to use X-rays to remove unwanted hair from women’s faces, and had many clients.

Dramatic use is made of early responses to X-rays in Thomas Mann’s novel The Magic Mountain (1924), which is set in a Swiss sanatorium in the years before the First World War. A student, Hans Castorp, has come to the sanatorium to visit his cousin Joachim, a patient there. The resident physician, Hofrat Behrens, takes him to the room containing the X-ray apparatus, where Joachim is to be examined.

They heard a switch go on. A motor started up, and sang furiously higher and higher, until another switch controlled and steadied it. The floor shook with an even vibration. The little red light, at right angles to the ceiling, looked threateningly across at them. Somewhere lightning flashed. And with a milky gleam a window of light emerged from the darkness: it was the square hanging screen, before which Hofrat Behrens bestrode his stool, his legs sprawled apart with his fists supported on them, his blunt nose close to the pane, which gave him a view of a man’s interior organism.

‘Do you see it, young man?’ he asked. Hans Castorp leaned over his shoulder, but then raised his head again to look toward the spot where Joachim’s eyes were presumably gazing in the darkness, with their gentle, sad expression. ‘May I?’ he asked.

‘Of course,’ Joachim replied magnanimously, out of the dark. And to the pulsation of the floor and the snapping and crackling of the forces at play, Hans Castorp peered through the lighted window, peered into Joachim’s empty skeleton. The breastbone and spine fell together in a single dark column. The frontal structure of the ribs was cut across by the paler structure of the back. Above, the collar bones branched off on both sides, and the framework of the shoulder, with the joint and the beginning of Joachim’s arm, showed sharp and bare through the soft envelope of flesh. The thoracic cavity was light, but blood vessels were to be seen, some dark spots, a blackish shadow.

‘Clear picture,’ said the Hofrat … ‘Breathe deep,’ he commanded. ‘Deeper! Deep, I tell you!’ And Joachim’s diaphragm rose quivering, as high as it could; the upper parts of the lungs could be seen to clear up, but the Hofrat was not satisfied. ‘Not good enough,’ he said. ‘Can you see the hilus glands? Can you see the adhesions? Look at the cavities here, that is where the toxins come from that fuddle him.’ But Hans Castorp’s attention was taken up by something like a bag, a strange, animal shape, darkly visible behind the middle column, or more on the right side of it – the spectator’s right. It expanded and contracted regularly, a little after the fashion of a swimming jelly-fish.

‘Look at his heart,’ and the Hofrat lifted his huge hand again from his thigh and pointed with his forefinger at the pulsating shadow. Good God, it was the heart, it was Joachim’s honour-loving heart, that Hans Castorp saw!