The 8th of November 1895, will forever be recorded in the history of science. On this date, X-rays, a previously unknown form of radiation, were discovered by the German physicist Wilhelm Conrad Röntgen [Figure 1].[1,2,3,4,5,6,7,8] The extraordinary discovery of this wonder rays has created an amazing step in the history of health sciences, which made the structures inside the body visible without the body being cut opened. The present article examines the momentous discovery of X-rays, the invisible short-wavelength electromagnetic radiation.
An illustrious man, Röntgen, was born on Thursday, 27th of March 1845, in Lennep, a small town on the Lower Rhine River at the heart of the industrial region of Germany, which today is a suburb of Remscheid. His father, Friedrich Conrad Röntgen, a cloth merchant, was also born in Lennep, while his mother Charlotte Frowein was from Amsterdam. Young Röntgen grew up in very happy surroundings. He was a very distinguished and interesting personality since childhood. He acquired a taste for art and beauty at a very young age.[1,6,7] He was a nature lover, fond of roaming in the forests, and was also a great mountaineer due to which he faced dangerous situations many times. The house in which Röntgen was born [Figure 2] is now part of the Röntgen museum with a large library. In March 1848, many European countries including Germany, were shattered by revolution. So the family moved to Apeldoorn, in the Netherlands, where young Röntgen was raised.
From 1861 to 1863, he attended school in Utrecht where he was not a particularly brilliant student. While in school, a curious event took place that had a considerable impact on his schooling life. He was expelled from school for refusing to reveal the identity of a classmate guilty of drawing a caricature of one of their teachers. Not only was he expelled, subsequently, he could not gain admittance to any other Dutch or German high school.[7,9] Later, he passed the examinations at the Federal Polytechnic Institute in Zurich and began studying mechanical engineering. On June 12th , 1869, Röntgen obtained his Ph.D. from the University of Zurich where he was a favorite student of Professor August Kundt. Röntgen had married Anna Bertha Ludwig from Zurich, Switzerland, in 1872.[1,3,5,7,10] She was the niece of the famous poet Otto Ludwig. They had no child but adopted the 6-year-old child of Bertha's brother.
Röntgen joined the University of Strassberg as a lecturer, at the age of 24. He became a professor at the Academy of Agriculture at Hohenheim, Württemburg, in 1875. The physics chair at the University of Würzburg was obtained by him in 1888, and in 1900, on a special request from the Bavarian government, he joined the University of Munich. Later he was supposed to join the Columbia University in New York; however, he had to change his plans due to World War I and he continued in the University of Munich till the end of his career. It was at the University of Wurzburg, where he stayed for 12 years and made the momentous invention of the wondrous rays. Röntgen's first study on the specific heat of gases was published in 1870, followed by a paper on the thermal conductivity of crystals. His research also examined the influence of pressure on the relative indices of various fluids, the characteristics of quartz, and the modification of the planes of polarized light by electromagnetic influences.[2,5] However, Röntgen got veritable recognition mainly because of his discovery of X-rays. Röntgen died in Munich on February 10th , 1923, that is 4 years after the death of his wife, from carcinoma of the intestine.[1,2,7] As per his will, following his demise, all his personal and scientific correspondence were destroyed.[2,7,11]
The Path Towards the Discovery
The story of the discovery of 'X-rays', as Röntgen called them, has been told in countless variations. Inevitably, many fables are spun about inventions of great significance, and the discovery of X-ray is no exception to that. The most popular story, perhaps, is that of the book and the key. It is presented in the following form: Röntgen once placed an experimental electric tube upon a book beneath which was a photographic plate. Sometime later he used the plate in his camera and was puzzled upon developing it, to find the outline of a key on the plate. He searched through the same book and discovered a key between the pages of it. The strange 'light' from the glass tube had penetrated the pages of the book; thus, the X-ray was discovered. Since Röntgen ordered, in his will, that all of his laboratory notes be destroyed, the actual sequence of events have been reconstructed from his personal communication records and from the accounts of his friends and biographers.
In 1895, Röntgen was studying the external phenomena that accompany the passage of an electric current through a rarified gas within various types of vacuum tubes. Previous research using the same tubes, had been carried out by Varley, Plucker, Hittorf, Hertz, Goldstein, Guericke, Lenard, and Crookes. The name cathode ray was coined by Goldstein to denote the electron beam generated in highly rarefied gases within vacuum tubes by using Ruhmkorff induction coils. Röntgen, in his communications, acknowledged the work of several renowned scientists of his time. Ruhmkorff, who also built musical instruments, later invented electromagnetic devices, such as the induction coil. Hittorf studied cathode rays and developed a tube with a more optimal vacuum than those previously available. William Crookes built a wide variety of vacuum tubes designed for research. His contribution was so great, that even in his lifetime the term Crookes tube was used for any type of vacuum tube. Röntgen also referred to Lenard, who was awarded a Nobel Prize for his ingenious work on cathode rays. In 1892, he built a tube with the rays directed towards a thin aluminum window, enabling, for the first time, the study of cathode rays outside the tube in which they were produced. Röntgen's work using cathode rays led him to the discovery of the previously unknown X-ray radiation.
Like all research scientists, Röntgen was on the lookout for new phenomena, and was following the path of all his illustrious predecessors when he made the discovery. In early November, he was repeating an experiment with various vacuum tubes, mainly the Hittorf-Crookes and Lenard (with a thin aluminum window) tubes. A cardboard covering was added to protect the aluminum from damage by the strong electric field necessary to produce cathode rays. Although the cardboard covering prevented light from escaping, Röntgen observed that the invisible cathode rays caused a small cardboard screen painted with barium platinocyanide to fluoresce.[1,7] Röntgen thought that the Hittorf-Crookes tube with a much thicker glass than the Lenard tube, might also cause this fluorescent effect.[1,7]
It has been proposed that Röntgen was using Hittorf-Crookes tube while making the marvelous discovery. He noted the fluorescence of the glass walls of the tube after energizing the cathode ray tube. Later he wrapped the entire tube in black cardboard and ensured that the cardboard shielding was adequate. Despite efficient shielding, a faint glimmer was visible from the barium platinocyanide fluorescent screen, which was more than a 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 or the cathode rays, as they did not travel very far through air. He realized that he was experiencing the new, previously unknown form of radiation that led to the fluorescence of barium platinocyanide screen that he kept some distance away.[1,2,3,4,5,6,7,8,9,10]
The study of this 'new light' was carefully planned. At one point, while investigating the ability of various materials to stop these new rays, he happened to bring his hand into the path of this new light and was startled to see the ghostly image of the bones of his hand on the barium platinocyanide screen. Thus, he also discovered that the new rays could pass through human tissue, leaving the bones visible.[1,5,7]
After his first observation, Röntgen feverishly researched the nature of this radiation for several weeks. He seldom emerged from the laboratory, had his meals there, and even slept there to investigate new ideas concerning his work. Replacing the fluorescent screen with a recording photographic plate was one of his first important successful steps.[1,5,7] Over the following weeks, Röntgen worked very hard in his laboratory, investigating this phenomenon using photographic plates to record images. On the evening of December 22, 1895, he photographed his wife Bertha's hand using the new rays. Following a 15-minute exposure, an image showing the bones of her hand and the two rings she was wearing appeared, which became the first ever radiograph of a human being [Figure 3].[1,2,4,5,7] When she saw her skeleton she exclaimed, "I have seen my death!";  thus, the use of radiology as a medical diagnostic technique was born.
Due to their unknown nature, Röntgen called them X-rays, because in mathematics 'x' is commonly used to indicate an unknown quantity. Although the new rays would also became known as Röntgen rays, he always preferred the term 'X-rays'.[1,5,7] By early 1896, X-rays soon became a part of medical investigations and were being used clinically in the United States for bone fractures and gunshot wounds. Continued with the excitement surrounded the new technology, the first X-ray department in the world had been set up at the Glasgow Royal Infirmary. The head of the department, Macintyre, produced a number of significant radiographs showing a penny in the throat of a child and the X-ray of a kidney stone. Similarly, Edwards discovered a needle embedded in a woman's hand using X-rays.
Within 2 weeks of Röntgen's first publication in February 1896, health professionals started making radiographs of the teeth. It is difficult to say who made the first dental radiograph. Koenig in Frankfurt made 14 radiographs during that period, while Walkhoff, a dentist in Brunschweig, took a radiograph of his own molar teeth with the help of a friend after a 25-minute exposure and noted the difficulty in remaining motionless for such a long time. The image could show only the white outlines of the teeth against a dark background. However, an English dentist in Sheffield, Harrison, made a dental radiograph after a 10-minute exposure that showed the outlines of the pulp chamber. Morton was the first to announce that radiographs of the teeth were possible. Kells, a dentist, is acknowledged for his significant contributions to dental radiography, because he designed his own dental X-ray apparatus in the 4 months after the discovery of X-rays, which drew worldwide attention. In an article published in 1899 in the journal, Dental Cosmos, he mentioned the importance of keeping the film and object at right angles to the source. He is also credited with educating dental professionals in many different applications of X-rays to dental practice.
By the time, X-rays became a part of investigations, it was established that the frequent exposure to X-rays could be detrimental to health and there was a need to find special measures to overcome them. Unfortunately, Kells was one of the early sufferers of the harmful effects of X-rays; his death made dental professionals aware of radiation-related injuries that accompany the use of X-rays. Röntgen died of carcinoma of the intestine. Notably, It is believed that his carcinoma was not a result of his work with ionizing radiation because he was one of the few pioneers in the field who routinely used protective lead shields.[2,7,11] Surprisingly, by the early 1900s, it was revealed that the damaging qualities of X-rays could be exploited to treat cancers and skin diseases. This has given rise to another era in the field of radiology.
It is interesting to note that 1 year before Röntgen made his discovery, he gave an address as the President of Würzburg University and in this speech he quoted some of the thoughts of one of his predecessors, Kircher, as: "Nature often reveals the most astonishing phenomena by the simplest means, but these phenomena can only be recognized by persons who have sharp judgment and the investigative spirit, and who have learned to obtain information from experience, the teacher of all things." How applicable this quote was in the case of his own discovery.
Spreading the News
"The inner structures of the body could be seen without the necessity of surgery" has created a stir in the entire world. The news of Röntgen's discovery spread all over the world with astonishing speed. The new 'wonder rays' appeared in rapid succession in the press around the world. X-ray machines soon became part of theater shows. The cartoons and poems that appeared in many popular magazines and newspapers formed an interesting part of the early printed records of the discovery, and gave a good idea of its reception by the general public.[1,7]
Röntgen wrote a short manuscript entitled "A New Kind of Rays," after 7 weeks of hard work. Röntgen wanted the manuscript to be published prior to its oral presentation. He sent an offprint of the manuscript to his old friend Exner, who showed it to Lecher, who then wrote the first article on the discovery which appeared in the Sunday edition (January 5, 1896) of the Vienna newspaper, Die Presse. On January 23, 1896, an English translation appeared in the Nature, and 2 weeks later in the Science.
On January 13, 1896, Röntgen presented his work to Emperor Wilhelm II in Berlin. The modest discoverer spoke for the first and only time on his 'New Kind of Ray,' before the Physical Medical Society of the University of Würzburg. When he entered the auditorium, a veritable storm of applause arose, which was repeated many times during the evening. Röntgen modestly began to talk about his work. He then mentioned the investigations with cathode rays made by Hertz, Lenard, and others, and said that his experiments along the same lines resulted in his discovery. He related how he first observed the fluorescence of a small piece of paper painted with barium platinum cyanide and how he quickly found out that the cause for this fluorescence came from the carefully covered Hittorf tube itself, and not from any other part of the high-tension circuit. Then, he used a photographic plate and the experiment was successful. After this, Röntgen made an X-ray picture (radiograph) of Albert von Kolliker's hand at a public lecture [Figure 4].[1,5,7,11]
Later, a detailed description of Röntgen's discovery was given by Thompson, a well-known physicist and president of the British Röntgen Society at a meeting held on November 5, 1897 in London. This description was published in November, 1897, in the second volume of the world's first journal of radiology, the English Archives of Skiagraphy, which was later called the Archives of the Röntgen Ray.
Honors and Awards
Röntgen was honored in an unprecedented way worldwide. In several cities, streets were named after him; he was awarded numerous honorary memberships, medals and other decorations. But being strikingly industrious and reticent, he never accepted these accolades in person. He did travel to Stockholm, however, to accept the first Nobel Prize in Physics in 1901. The award was given "in recognition of the extraordinary services he has rendered with the discovery of the remarkable rays subsequently named after him." [1,5,7] It is not known clearly about which of the various tubes Röntgen actually used for the discovery, although literature suggests it would be Hittorf-Crookes tube. Lenard, however, considered himself 'the mother of X-rays', while Röntgen was 'the midwife who happened to deliver the child.' Certainly, both scientists were nominated for the first Nobel Prize in Physics in 1901, however, the Royal Academy of Science decided to award the prize to Röntgen alone. Röntgen donated the prize amount to his university. After his discovery the University of Würzburg awarded an honorary Doctor of Medicine degree to Röntgen. The element number 111 was named Röntgenium (Rg) in his honor, in November 2004.
Despite all this, he remained a modest, diligent man, and continued his work.[1,5,7] He had many opportunities to make money and many avenues towards a lavish life. However, he turned down all the chances to make financial profit from his discovery. Levy, a representative of a well-known German company, was sent to Röntgen to negotiate a contract for the industrial exploitation of his current and future discoveries. Levy recalled Röntgen'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, licenses, contracts or be controlled by one group." 
Echoes Following the Discovery
In retrospect, it is apparent that many scientists unknowingly encountered X-rays in the course of their experiments. For example, Crookes complained to his supplier of photographic plates, about fogged and blackened plates in unopened boxes. Another well-known case of unintentional experimentation with X-rays while carrying out studies with Crookes tubes, was reported by Goodspeed. It was only after the publication of Röntgen's paper that the team was able to explain the cause of the strange shadowy pictures they had observed 6 years earlier. Thus, although many scientists before him had made similar observations, until Röntgen's announcement was made, they neither understood them nor found them of significance. The idea that Röntgen happened to notice the barium platinocyanide fluorescent screen misrepresents his investigative powers. Remarkably, the presence of a barium platinocyanide fluorescent screen in his experimental set-up was not accidental. He had planned to use the screen in the next step of his experiment. Röntgen fully understood the reasons underlying his observation. It was Röntgen's scientific background, keen observation, accuracy of his critical judgment, and his great experimental skill that resulted in the discovery. Unfortunately, even during his lifetime, tales that the discovery was more-or-less accidental were prevalent, despite the fact that Röntgen not only observed the fluorescent light on a barium platinocyanide screen but also demonstrated its nature with such flawless experimental techniques that no publication of equivalent importance was made by any other scientist until 1905, when Charles C Barkla from Liverpool discovered the so called 'characteristics' of X-rays.[1,2,5,7] Between 1895 and 1897, a total of three papers were published on X-rays by Röntgen.[1,2,3,4,5,7]
The statement by the philosopher, Münsterberg, repudiated the myth that Röntgen's discovery of X-rays was entirely accidental: " Suppose chance helped, there were many galvanic effects in the world before Galvanis saw by chance the contraction of a frog's leg on an iron gate. The world is always full of such chances, and only the Galvanis and Röntgens are few." [1,7] Röntgen`s discovery of X-rays was the final step in a brilliant and logical correlation of a multitude of facts that had been disclosed by many scientists. Today, Röntgen is considered as the father of diagnostic radiology.
It is difficult to appreciate fully the genius of the man who made this remarkable discovery and beneficently gave it to the world. Remarkably, Röntgen was not only intellectual but also a very inspirational personality. In spite of his splendid contribution to the mankind, he remained a courteous, dignified, and modest man. He was considerate, amiable, a good listener and continued with his love toward nature. Röntgen is credited with three standard components of X-ray investigation that are in current use. They are the fluorescent screen, the photographic plate, and the prototype of the ionization chamber method. The potential of X-rays for medical diagnosis was recognized immediately.
In the 100 years since the discovery of X-rays, several museums have been erected to collect and preserve the X-ray record and the heritage of Röntgen. Several books and review articles have been published that overviewed the developments within the various fields of technical and life sciences based on his discovery of X-rays.[12,13,14] Röntgen's discovery had an enormous impact on the world, both at that time and today.
The centennial year of Röntgen's X-ray discovery, 1995, was an occasion for radiologists and other members of the health-science community to tell the public about the accomplishments of radiation sciences in the healthcare field during the previous 100 years. Many conferences with invited speakers in various subfields of X-ray activities were organized to celebrate the discovery.
Imagine the world without this unprecedented discovery made by the great Röntgen. Radiologists and healthcare professionals all over the world have a debt of gratitude to the great German scientist for his vital discovery to mankind. Although significant progress has been made in the field of radiology, this has been built as further development of Röntgen's basic approach. Röntgen's pioneering discovery opened up new vistas in research; the work of a number of Nobel laureates has hinged on the properties of X-rays. Röntgen's discovery has been fundamental to development of modern physics; his name and his work will live forever even though we are observing the era of demise of the X-ray films.
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