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Popular Science Monthly/Volume 73/December 1908/The Story of Professor Rontgen's Discovery

< Popular Science Monthly‎ | Volume 73‎ | December 1908



THE discovery of X-rays was announced by Professor Röntgen in December, 1895, in a communication to the Physico-medical Association of Würzburg. The date of the discovery is commonly thought to be November, 1895. As a matter of fact, the first X-ray photograph was made about two years before that time, and the accidental production of this photograph was the starting point of a series of investigations which continued for more than two years before the public announcement was made. The story was told to me by Dr. T. S. Middleton, now a physician in Chicago, who was a research student under Professor Röntgen during a period of four years, including the time when the great discovery was made.

Professor Röntgen is a man who works unceasingly as a teacher and in research, a man who brings to his students the inspiration of genius. Like Edison, he would often forget to eat were he not reminded by friends of his need of food.

He was working with cathode rays and, being an expert glass blower, prepared his own tubes. He had a habit of using his lungs as an air-pump in exhausting his tubes. Long practise had developed an athletic pair of lungs, so that he was able in this manner, aided by the increase in vacuum due to the electric discharge, to produce a vacuum sufficiently high for the production of the cathode rays. The first X-ray tube was exhausted in this way. This tube was blown to form a large bulb at the middle and bent to form a letter S at either end.

The electrodes being at the ends, the cathode rays would have to traverse the bends of the tube. Röntgen regarded the cathode rays as streams of electrified particles and believed that friction would be developed as these particles streamed past the bends of the tube. He expected this friction to result in new phenomena.

On a flat-topped desk in Professor Röntgen's private office lay an unassorted heap of books, glass tubes, photographic plate holders, platinum and aluminium electrodes, and what-not, such an unassorted heap as is likely to accumulate on the desk of a busy man. In this confusion it happened that a large book which the professor had been reading lay on a photographic plate holder. In the book lay a key serving as a bookmark. The use of a flat key as a bookmark is a peculiar habit of Professor Röntgen's, a habit which leads often to the finding of lost keys by shaking open the books on his desk. The professor was working with the Crookes tube referred to above, observing the beautiful yellowish-green fluorescence which characterized this particular tube, when his wife came to call him to lunch. Laying the tube, still glowing, on the book he obeyed her summons.

Now Professor Röntgen is an enthusiastic amateur photographer, in fact out door photography is his recreation. Returning from lunch, he took the plate holder which had lain under the book, with other plate holders, and made several outdoor exposures. On developing the plates a shadow picture of a key appeared on one of them. Much puzzled, he showed the negative to some of his students, asking them to suggest some explanation of the mysterious key. None of their suggestions proved satisfactory, and he was up early the next morning searching for a solution of the mystery.

He determined to repeat precisely the operations of the preceding day and, remembering the positions of the glowing tube, the book, and the plate holder, he placed them as before, leaving them for the same length of time as on the preceding day. On developing the plate, the image of the key again appeared. The key was found in the book but the mystery was not solved. Here was indeed a strange thing. Of course it was known that the cathode rays would affect a photographic plate, but here between the plate and the source of the rays were a book and the hard-rubber slide of the plate holder, both of which are impervious to light, and the cathode rays were confined by the walls of the tube.

Röntgen continued his investigations and found that the rays from his tube would penetrate other objects, but in different degrees, and because of this difference in transparency he could obtain shadow pictures of many interesting objects.

The fluorescence of his tube suggested to him that other substances than glass might be caused to fluoresce by the radiation from the tube. An interesting field was opened before him. Other research was suspended. Visitors were excluded, and with his research students the work was continued. Fluorescent liquids were tested, one of the first being a solution prepared from the horse-chestnut. The number of fluorescent substances tested, including liquids and solids, was not less than fifty. One of these was barium platino-cyanide, the fluorescence in this case being caused by the newly-discovered radiation acting through a black cardboard. Convinced by a long series of observations of the photographic and fluorescent effects obtained from his tube that he had discovered a new form of radiation, Professor Röntgen made his public announcement.

In answer to my question regarding the date of the incident narrated above, which marked the beginning of the great discovery, Dr. Middleton said that he had made no effort to remember the date. "Was it earlier than November, 1895," I asked? "Yes. It was at least two years earlier" and, recalling some incidents to aid in fixing to date, he continued, "It was soon after the opening of the autumn semester. It could not have been later than October, 1893. Possibly it was in 1892."

Professor Röntgen, being a reticent man, has not given to the world the details of his discovery. His announcement was that of results arranged in logical not chronological order. This narrative from an authentic source fixes the date of the discovery of X-rays at least two years earlier than that which is commonly accepted. Moreover, it shows us the great German professor in the light of a man who carefully and patiently worked out his problem to his own satisfaction before announcing his discovery to the world.