Textbook of Radiology

W.C Roentgen and the discovery of X-rays


In today's competitive academic climate, W.C. Roentgen would probably not even attain a place at university, let alone one at medical school. He certainly would not be accepted onto one of the more prestigious radiological programmes, and his pioneering paper "On a New Kind of Rays" published in an undistinguished German journal- a journal without peer-review and devoid of any academic impact - would probably remain unnoticed in the world of science. Fortunately, however, one hundred years ago things were different ...

On the occasion of both the 150th anniversary of W.C. Roentgen's birth and the centennial celebrations of his momentous discovery, you are cordially invited to join me on a short historical journey.

Wilhelm Conrad Roentgen was born on Thursday, March 27th, 1845 in Lennep (today a suburb of Remscheid, Germany) to Friedrich Conrad Roentgen, a distinguished cloth manufacturer and merchant, and his wife Charlotte Constanze, née Frowein. The house in which Roentgen was born still stands today and now forms part of the Roentgen Museum, where it houses a large library (Fig. 1). Visitors are able to study there in the peace and quiet afforded by a secluded district town.

In 1848, many European countries, including Germany, were shattered by revolution. W.C. Roentgen's parents sold their house in Lennep and emigrated to Apeldoorn (The Netherlands), because Mrs. Roentgen was of Dutch origin. Wilhelm attended primary school there, and then a private boarding school, until 1861, when he left home to continue his studies in Utrecht. A somewhat curious event took place there, which was to have a considerable impact on his future life. What would be a harmless student prank by today's standards caused a "consilium abeundi" - in other words, he was expelled from school. What actually happened has never been made completely clear.

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Figure 1.
Wilhelm Conrad Roentgen's birthplace at Remscheid-Lennep, Germany.


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Figure 2.
Wilhelm Conrad Roentgen during his schooldays in Holland


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Figure 3.
Wilhelm Conrad Roentgen during his time in Würzburg.

According to Glasser's biography, one of Roentgen's classmates had drawn a caricature of the teacher on the firescreen when the teacher arrived unexpectedly early and discovered the picture. He became very angry and asked Roentgen to name the culprit, which Roentgen refused to do. The teacher threatened him with suspension and eventually succeeded in having him expelled.

On advice from friends of his father, Roentgen revised at home in preparation for the entrance examinations to university. Unfortunately, in the decisive examination one of the examiners was a teacher who had been involved in the suspension proceedings at his former school, with the result that Roentgen failed to secure a place. It seemed that the road to university was permanently blocked (Fig. 2).

In 1862 W.C. Roentgen enrolled at the Utrecht Technical School, a private institution which prepared students for entrance into technical high school by way of a two-year course. Then, in 1865, he was enrolled for a short time as a visiting student in the Department of Philosophy at the University of Utrecht. In November 1865 he moved to Zürich (Switzerland) and the Polytechnic of Zürich (today Eidgenossische Hochschule, ETH) which accepted students such as Roentgen who did not have a normal school leaving certificate, after a demanding admission test.

While in Zürich, Roentgen studied mechanical engineering, seriously and successfully, and in 1868 he received his diploma in that subject. He then continued his studies, in close cooperation with August Kundt, who, at the age of just 29 years, was Professor of Physics at the ETH. On June 12, 1869, Roentgen obtained his Ph.D. degree from the University of Zürich following submission of his thesis entitled "Studies on Gases".

At the age of 24, W.C. Roentgen became Professor Kundt's assistant in the Department of Experimental Physics. Two years later, in 1870, Kundt accepted the Chair of Physics at the University of Würzburg, and Roentgen followed him there.

Würzburg University, however, considerably hindered the furtherance of Roentgen's academic career. Acting under Bavarian law, it barred his academic promotion because of both his lack of a school leaving certificate and his lack of the requisite training in classical languages, despite Kundt's efforts on his behalf.

Roentgen's situation improved in 1872, nevertheless, when Kundt was invited to the newly founded German University of Strassbourg (now in France) and took his assistant with him once again. This new university was less dogmatic and assisted Roentgen in his scientific career: in 1874, Roentgen was promoted to "Privat-Dozent" and was appointed as a lecturer at Strassbourg University. 

Only one year later, in 1875, at the age of 30, Roentgen was called to the Agricultural Academy of Hohenheim (near Stuttgart, Germany) as Professor of Physics and Mathematics.

He was not happy there, however, because the institute was poorly equipped and this prevented him from carrying out his scientific work. As a result he readily accepted an offer of the position of Associate Professor with his former teacher Kundt at the University of Strassbourg, and returned there in 1876.

Three years later he was offered the Chair of Physics at the University of Giessen (Germany). A new institute was built for him there, allowing him to carry out a series of important experimental studies which further strengthened his position as one of the leading physicists in Germany at that time. In 1886 he received an offer from the University of Jena (Germany) and, in 1888, another from the University of Utrecht (The Netherlands), both of which he declined.

In 1888, however, Roentgen received an offer which he could not refuse: the same University of Würzburg, which had previously obstructed his academic career now offered him the position of full professor and director of its highly esteemed and well equipped Physics Institute. He accepted the appointment and moved back to Würzburg where, in 1894, he received the ultimate academic accolade in his election as Rector. On November 8, 1895 he discovered a new kind of rays, a discovery which laid the foundations for the development of our medical speciality and which led to a phenomenal expansion in research in the fields of physics, technology and astronomy (Fig. 3).

A new kind of rays

The story of the discovery of "X-rays", as Roentgen called them, has been told in countless variations. This is due mainly to the fact that in his will Roentgen requested that on his death all his papers, including his laboratory books, be burned, unread. There is thus much room for speculation and relatively little safe historical ground.

Unfortunately, the limited scope of a short historical note such as this prevents us from detailing the various traditions in any depth. Suffice it to say that even during Roentgen's lifetime the story was already current that the discovery was made more or less accidentally, despite the fact that it was Roentgen who had not only observed the fluorescent light on a barium platinocyanide screen but who also demonstrated its nature with such flawless experimental technique that, following the publication of his three papers entitled "On a New Kind of Rays", no publication of equivalent importance was made by any other scientist until 1905 when Charles C. Barkla from Liverpool discovered the so called "characteristic" X-ray.

The prevailing tenor of contemporary discussion regarding the circumstances of Roentgen's discovery is best encapsulated in a comment made by the contemporary philosopher Münsterberg of Harvard University who on January 15th, 1896 wrote in a report to Science:

"Suppose chance helped. There were many galvanic effects in the world before Galvani saw by chance the contraction of a frog's leg on an iron gate. The world is full of such chances and the Galvanis and Roentgens are few."

In his original communication Roentgen reported the experimental setup and his observations (Roentgen 1895):

"If one passes the discharges of a fairly large Ruhmkorff induction coil through a Hittorf vacuum tube, a sufficiently evacuated Lenard or Crooke’s tube, or a similar apparatus, and if one covers the tube with a rather closely fitting envelope of thin black cardboard, one observes in the completely darkened room that a piece of paper painted with barium platinocyanide lying near the apparatus glows brightly or becomes fluorescent with each discharge, regardless of whether the coated surface or the other side faces the discharge apparatus. The fluorescence is still visible at a distance of 2 m from the apparatus ..."
(Roentgen's original communication, translated by Otto Glasser.)

In the communication Roentgen acknowledges the achievements of several renowned scientists of his time: H.D. Ruhmkorff (1803-1877) originally built musical instruments. In later life he moved to Paris and became famous for the invention of electromagnetic devices such as the induction coil alluded to in the above quotation. J. W. Hittorf (1824-1914), Professor of Physics and Chemistry at the University of Münster, Germany, studied cathode rays and developed a tube with a vacuum more complete than had hitherto been available. Sir William Crookes (1832-1919) built a wide variety of vacuum tubes designed for research into "radiant matter". He was of such renown that even in his lifetime the term "Crookes tube" was used to refer to any kind of vacuum tube. Lastly, Roentgen refers to P. Lenard (1862-1947), who was awarded the Nobel Prize in Physics in 1905 for his ingenious work on cathode rays. In 1892 he built a tube with the rays directed toward a thin aluminium window, enabling for the first time the study of cathode rays outside the tube in which they were produced. It is not known which of these various tubes W.C. Roentgen actually used; in his original communication he merely mentioned the three different types in a general way. Lenard, however, considered himself to be "the mother of X-rays" while Roentgen was "the midwife who happened to deliver the child". Certainly, both scientists were nominated for the first Nobel Prize in Physics in 1901; and the committee recommended that the prize should be divided equally between Roentgen (München), and Lenard (Kiel). In the event, the Royal Academy of Science did not follow this recommendation but decided to award the prize to Roentgen alone. In 1896, however, the two scientists were jointly awarded the Rumford Medal of the Royal Society of London. Neither man actually travelled to London to receive his prize, though, and as a result, the two men did not meet on this occasion, nor, indeed, on any other.

The moment of discovery

Owing to the fact that W.C. Roentgen ordered that all his laboratory notes be destroyed, we must attempt to reconstruct the precise sequence of events from his own first communication and from the accounts of his friends and biographers.

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Figure 4.
The old Department of Physics, University of Würzburg, where W C. Roentgen discovered the rays.

It was late afternoon on Friday, November 8th, 1895, W,C. Roentgen preferred to work alone in his laboratory, and on a Friday he could be fairly certain that nobody would disturb him. He had been conducting a series of experiments on cathode rays using a Lenard's tube - the one with the thin aluminium window - but, apparently, that night he decided to use a Rittorf or Crookes tube instead, without such a window (Fig. 4).

After energizing the cathode ray tube he noted the well known phenomenon of fluorescence of the tube's glass walls. He had darkened the room and was investigating the fluorescence of barium platinocyanide pasted onto a piece of cardboard. In order to avoid interference with fluorescence originating from the glass walls of the tube he wrapped the entire tube in black cardboard. After energizing the tube again he made sure that the cardboard shielding was adequate. Despite efficient shielding, however, a faint glimmer was visible from the barium platinocyanide screen, which was more than one meter away from the tube. He repeated the experiment several times and established beyond doubt that this phenomenon could not be due to ordinary light reflections, nor due to the cathode rays, because they did not travel so far through air.

Over the following seven weeks Roentgen worked very hard in his laboratory investigating this phenomenon. It is said that he even ate his meals in the laboratory and had his bed moved there so that he could work without interruption.

Roentgen had already proved his capability as an experimental physicist, and the experimental set-up for the study of the "new light" was carefully planned. To document his observations he used photographic plates. On the evening of December 22, 1895, he asked his wife Bertha to let him photograph her hand using the new rays. After a fifteen minute exposure the first radiograph of a human being was obtained showing clearly the bones of the hand and the two rings she was wearing. This date is the true birthday of radiology as a medical speciality (Fig. 5).

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Figure 5.
Radiograph of the hand of Bertha Roentgen, taken December 22nd, 1895.


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Figure 6.
First 8-ray shadow picture, taken accidentally by Arthur w: Goodspeed at the University of Pennsylvania on February 22nd, 1890


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Figure 7.
Roentgen's first communication. First page of the handwritten manuscript.

In retrospect it is apparent that many scientists unknowingly encountered X-rays in the course of their experiments. For example, Crookes complained to Ilford, his supplier of photographic plates, about fogged and blackened plates in unopened boxes. The company replaced the plates but observed that the damage must have occurred in the physicist's institute, since no other complaints had been received.

Another well-known case of unintentional experimentation with X-ray is that which was reported by Professor A.W. Goodspeed of the University of Pennsylvania as occurring on February 22, 1890, when carrying out studies with Crookes tubes. It was only after the publication of Roentgen's paper that those involved were able to explain the cause of the strange shadowy pictures they had taken six years earlier (Fig. 6).

Spreading the news

We tend to assume that rapid communication is an achievement made possible only in this electronic age of satellites, fax machines and global computer networks; yet news of Roentgen's discovery spread all over the world with astonishing speed. After seven weeks of hard work W.C. Roentgen wrote a short manuscript entitled "On a New Kind of Rays. First Communication" and handed it to the secretary of the Würzburg Physical Medical Society. Because the society did not meet during the Christmas holiday, Roentgen asked that the manuscript be published prior to its oral presentation, which was scheduled for January 23rd, 1896. The secretary agreed and the manuscript was published in the "Sitzungsberichte der Physikalisch-Medizinischen Gesellschaft in Würzburg" (S. 132-141, Band 137, 1895) (Fig. 7).

Three days later, on January 1st, 1896, Roentgen had already obtained the offprints of the manuscript and had sent them, together with a few illustrative pictures, to his scientific colleagues and friends as a New Year Greeting.

One of these offprints went to his old friend Prof. Exner, whom he had known since college days in Zürich. Prof. Exner showed the manuscript and pictures to a group of physicists who were attending an informal scientific get-together at his home. One member of the group was Prof. Lecher from Prague who asked Exner if he might borrow the manuscript for a single day. Lecher's father was the editor of the Vienna newspaper "Die Presse", and it was he who wrote the first article on the discovery, which appeared in the Sunday, January 5th, 1896, edition of that newspaper. Owing to haste in the preparation of the article the name of the Würzburg Professor was misspelled as "Routgen". By the following evening, that of Monday, January 6th, 1896, the news had been announced to the whole world by cable from London. The press notice read:

"The noise of war's alarm should not distract attention from the marvellous triumph of Science which is reported from Vienna. It is announced that Professor Routgen (sic) of Würzburg has discovered a light which, for the purpose of photography, will penetrate wood, flesh and most other organic substances. The Professor has succeeded in photographing metal weights which were in a closed wooden case, also a man's hand, which shows only the bones, the flesh being invisible".

The London Standard printed the report on January 7th, 1896, adding the following remarks:

"The Presse assures its readers that there is no joke or humbug in the matter. It is a serious discovery by a serious German Professor".
While the first reports in New York were published on January 8th, 1896, the first report by the local newspaper, "Würzburger Generalzeiger", did not appear until January 9th, l896!

Thus, within ten days of the submission of the manuscript to a publisher the news had spread to the entire world - before, even, Roentgen had presented his findings to the scientific society. On January 23rd, 1896, an English translation appeared in Nature (London), and, two weeks later, in Science (USA).

On January 13th, 1896, Roentgen presented his work to Emperor Wilhelm II in Berlin and, on January 23rd, 1896, he eventually gave his oral presentation to the Würzburg Physical Medical Society.

The period following the discovery

W.C. Roentgen did not relish the sudden publicity he gained after the discovery of "a new kind of rays" which were named "Roentgen rays" following his oral presentation at Würzburg. He was awarded numerous honorary memberships, medals and other decorations but virtually never accepted these accolades in person. He did travel to Stockholm, however, to accept the first Nobel Prize in Physics in 1901, but did not give an official speech even on that occasion. He was honoured by the Prince Regent of Bavaria with the Royal Order of Merit of the Bavarian Crown which carried with it personal nobility. Roentgen accepted the decoration but refused the status of nobility, a gesture which was most uncommon in those days in Germany. He turned down several chances to make financial profit from his discovery. Mr. Levy, a representative of a well known German company, was sent to Roentgen to negotiate a contract for the industrial exploitation of his current and future discoveries. Mr. Levy recalled Roentgen's answer:

"He declared, however, that according to the good tradition of German University professors he was of the opinion that his discovery and inventions belonged to humanity and that they should not in any way be hampered by patents, licences, contracts or be controlled by one group."

In 1900, W.C. Roentgen became Professor of Physics at the University of Munich and Director of the new Physics Institute. He retired in 1920 and died, at the age of78, on February 10th, 1923, in Munich. His mortal remains were put to rest beside those of his wife and his parents in a cemetery at Giessen.


Peter Peters