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Early History of X Rays
by ALEXI ASSMUS
10 SUMMER 1995
The discovery of X rays
in 1895 was the
beginning of a
revolutionary change
in our understanding
of the physical world.
I
N THE WINTER of the year of his fiftieth birthday, and the year
following his appointment to the leadership of the University
of Würzburg, Rector Wilhelm Conrad Roentgen noticed a barium
platinocyanide screen fluorescing in his laboratory as he
generated cathode rays in a Crookes tube some distance away.
Leaving aside for a time his duties to the university and to his
students, Rector Roentgen spent the next six weeks in his labora-
tory, working alone, and sharing nothing with his colleagues.
BEAM LINE 11
Three days before Christmas he brought his wife into his laborato- ry, and they emerged with a photo- graph of the bones in her hand and of the ring on her finger. The Würzburg Physico-Medical Society was the first to hear of the new rays that could penetrate the body and photograph its bones. Roentgen delivered the news on the 28th of December 1895. Emil Warburg relayed it to the Berlin Physical Society on the 4th of Janu- ary. The next day the Wiener Press carried the news, and the day fol- lowing word of Roentgen’s discovery began to spread by telegraph around the world. On the 13th of January, Roentgen presented himself to the Kaiser and was awarded the Prussian Order of the Crown, Second Class. And on the 16th of January the The New-York Times announced the discovery as a new form of photography, which revealed hidden solids, penetrated wood, paper, and flesh, and exposed the bones of the human frame. “Men of science in this city are awaiting with the utmost impatience the arrival of English technical journals which will give them the full par- ticulars of Professor Roentgen’s dis- covery of a method of photographing opaque bodies,” The New-York Times began, and it concluded by pre- dicting the “transformation of mod- ern surgery by enabling the surgeon to detect the presence of foreign bodies.” (Jan. 16, 1896, p. 9) The public was enthralled by this new form of photography and curi- ous to know the nature of the new rays. Physicians put it to immediate use. Physicists sat up and took no- tice. The discovery of X rays was the first in a series of three discoveries
that jolted the fin- de-siècle disci- pline out of its mood of finality, of closing down the books with ever more precise measurements, of losing itself in de- bates over statistical mechanics, or of try- ing to ground all physical phenomena in mathematically precise fluctuations of the ether. All three discoveries, X rays, uranium rays, and the elec- tron, followed from one of the major experimental traditions in the second half of the nineteenth century, the study of the discharge of electricity in gases. All three contributed to a profound transfor- mation of physics. In the 20th cen- tury, the discipline has been ground- ed in the study of elementary particles. As with the invention of in- candescent light bulbs, the study of electrical dis- charge through gases was made possible by the development of improved vacu- um technology in the 1850s. Ear- ly on, English scientists were investigating the patterns of light and dark that ap- peared in sealed lead-glass tubes. The patterns in
Wilhelm Conrad Roentgen (1845–1923). (Courtesy of AIP Emilio Segré Visual Archives)
Forms of tube used by Roentgen in 1895–1896 for the production of X rays.
German Museum, Munich
BEAM LINE 13
refused the calls until the Universi- ty of Würzburg offered him the Directorship of their Physical Insti- tute. In 1894 he was elected Rector at Würzburg. In his inaugural ad- dress, given the year before his dis- covery of X rays, Roentgen stated that the “university is a nursery of scientific research and mental edu- cation” and cautioned that “pride in one’s profession is demanded, but not professional conceit, snobbery, or academic arrogance, all of which grow from false egoism.” * Roentgen’s pride could rest in the over forty papers he had published from Strasbourg, Giessen, and Würzburg. These early interests ranged widely—crystals, pyroelec- trical and piezoelectrical phenomena, and the effects of pressure on liquids and solids—but did not yet include electrical discharges in gases. He had taken his turn at measuring the specific heat ratios of gases using a sensitive thermometer of his own making. He was an exact experi- menter who often made his own apparatus—a skill learned during his training as an engineer in Zurich— and he was able to measure ex- tremely small effects, surpassing even Faraday’s measurement of the rotation of polarized light in gases. Roentgen turned to a new interest in October of 1895: the study of cath- ode rays. In the course of repeating the experiments of Hertz and Lenard, he happened to notice a glowing flu- orescent screen set off quite some distance from the Crookes’ tube he was operating. The screen sat much farther away than the six to eight
centimeters that Lenard had found to be the maximum distance for which cathode rays maintain their power to induce fluorescence. Roent- gen recognized the effect as wor- thy of his undivided attention and devoted the next six weeks to its uninterrupted study. Historians have speculated about why Roentgen was the first to rec- ognize the significance of this effect. The equipment, a cathode ray tube and a fluorescing screen, had been in use for decades. In 1894 J.J. Thomson had seen fluorescence in German- glass tubing several feet from the discharge tube. Others had noted fogged photographic plates. But before Lenard’s work, the object of study was always the effects inside the tube itself, and stray ultra-ultra- violet light could be used to explain the fogging of photographic plates. Lenard’s great interest was in prov- ing, in contradiction to the British, the ethereal nature of cathode rays, and he was the first to study the
Demonstration by Crookes that cathode rays travel in straight lines: a) cathode; b) aluminum cross and anode; d) dark shadow; c) fluorescent image.
Phillip Lenard, 1862–1947. (Courtesy of Ullstein Bilderdienst and the AIP Niels Bohr Library)
*Quoted in “Wilhelm Conrad Roentgen,” Dictionary of Scientific Biography (New York: Scribner’s, 1975), p. 531.
14 SUMMER 1995
effects of the rays in air or in a sec- ond glass tube into which he directed them. Roentgen, a meticulous and ob- servant experimenter, made the obvious tests on the new X rays: Were they propagated in straight lines? Were they refracted? Were they reflected? Were they distinct from cathode rays? What were they? Like the cathode rays, they moved in straight lines. Roentgen was unable to refract them with water and car- bon bisulphide in mica prisms. Nor could he concentrate the rays with ebonite or glass lenses. With ebonite and aluminum prisms he noted the possibility of refracted rays on a pho- tographic plate but could not observe this effect on a fluorescent screen. Testing further, he found that X rays could pass freely through thick lay- ers of finely powdered rock salt, electrolytic salt powder, and zinc dust, unlike visible light which, because of refraction and reflection, is hardly passed at all. He concluded that X rays were not susceptible to regular refraction or reflection. Roentgen found that the X rays originate from the bright fluores- cence on the tube where the cathode rays strike the glass and spread out. The point of origin of the X rays moves as the cathode rays are moved by a magnetic field, but the X rays themselves are insensitive to the magnet. Roentgen concluded that they are distinct from cathode rays, since Lenard’s work had shown that cathode rays passing through the tube maintained their direction but were susceptible to magnetic deflection. Roentgen justified calling the new phenomena rays because of the
O, Röntgen, then the news is true, And not a trick of idle rumour, That bids us each beware of you, And of your grim and graveyard humour.
We do not want, like Dr. Swift, To take our flesh off and to pose in Our bones, or show each little rift And joint for you to poke your nose in.
We only crave to contemplate Each other’s usual full-dress photo; Your worse than “altogether” state Of portraiture we barin toto!
The fondest swain would scarcely prize A picture of his lady’s framework; To gaze on this with yearning eyes Would probably be voted tame work!
No, keep them for your epitaph, these tombstone-souvenirs unpleasant; Or go away and photograph Mahatmas, spooks, and Mrs. B-s-nt!
—Punch, January 25, 1896
shadowy pictures they produce: bones in a hand, a wire wrapped around a bobbin, weights in a box, a compass card and needle hidden away in a metal case, the inhomo- geneity of a metal. The ability of the new rays to produce photographs gave them great popular appeal and brought Roentgen fame. Many arti- cles appeared in photography jour- nals, and The New-York Times in- dexed the new discovery under photography. Since the rays exposed photographic plate, the public as- sumed they were some form of light. The physicist Roentgen concurred. Accepting Lenard’s claim that cath- ode rays were vibrations of the ether, Roentgen compared the new rays to them and forwarded the opinion that the two were ethereal, although dif- ferent from visible, infra-red and ultra-violet light in that they did not reflect or refract. He suggested that cathode rays and X rays were longi- tudinal vibrations of the ether rather than transverse ones. Now that their existence was established, it was easy enough to experiment with the new X rays. Roentgen himself published only three papers on the subject, but oth- ers jumped quickly into the field. And not just physicists. Thomas Edison used modified incandescent light bulbs to produce the new rays. He boasted to reporters that any- one could make photographs of skeleton hands; that was mere child’s play. Within a month of Roentgen’s announcement doctors were using the X rays to locate bullets in human flesh and photograph broken bones. Dr. Henry W. Cattell, Demonstrator of Morbid Anatomy at the Univer- sity of Pennsylvania, confirmed their
Heinrich Rudolf Hertz, 1857–1894. (Courtesy of Deutsches Museum and AIP Emilio Segrè Visual Archives)
16 SUMMER 1995
Hermann Haga and Cornelius Werd, announced that X rays could be dif- fracted, and a Privatdozent at Göt- tingen named Arnold Sommerfeld carried out a mathematical analy- sis of diffraction to show that their results could be explained in terms of aperiodic impulses. In 1904, Charles Glover Barkla, a student of both Stokes and Thomson at Cam- bridge, showed that X rays were plane polarizable while experiment- ing with secondary and tertiary X rays. (These were produced by directing X rays against solids.) As X rays began to show, more and more, the properties of light, urani- um rays provided new mysteries. They themselves were composed of three sorts of distinct rays: α, β, and γ rays. What were these? Suddenly physics, which had seemed to some to be coming to a conclusion, was faced with unexplainable, qualita- tive discoveries. They were not “in the sixth place of the decimals,” as Michelson had predicted. At the international congress on physics, staged in Paris in 1900 by the French Physical Society, fully nine percent of the papers delivered were on the new ray physics. In 1899 Ernest Rutherford, another student of Thomson’s and the man who would become his successor as
WE WANT TO KNOW
If the Roentgen rays, that are way ahead, Will show us in simple note, How, when we ask our best girl to wed, That lump will look in our throat.
If the cathode rays, that we hear all about, When the burglar threatens to shoot, Will they show us the picture without any doubt, Of the heart that we feel in our boot.
If the new x-rays, that the papers do laud, When the ghosts do walk at night, Will show ’neath our hat to the world abroad How our hair stands on end in our fright.
If the wonderful, new, electric rays, Will do all the people have said, And show us quite plainly, before many days, Those wheels that we have in our head.
If the Roentgen, cathode, electric, x-light, Invisible! Think of that! Can ever be turned on the Congressman bright And show him just where he is at.
Oh, if these rays should strike you and me, Going through us without any pain, Oh, what a fright they would give us to see The mess which our stomachs contain!
—Homer C. Bennett, American X-ray Journal, 1897
director of the Cavendish Laboratory, had separated α rays, stoppable by metal foil or paper sheets, from the more penetrating β rays. In 1900, Rutherford had identified the βs as high-speed electrons: deflected in a magnetic field they showed the cor- rect charge-to-mass ratio. A third component of the uranium rays, undeviable and highly penetrating, was discovered by Paul Villard at the Ecole Normale Superieur in Paris. Rutherford named these γ rays. In her 1903 thesis Marie Curie made these comparisons: γ rays to X rays; β rays to cathode rays; and α rays to canal rays. (Canal rays were streams of positively charged molecules.) A few years later another story came out. The British scientist William Henry Bragg announced in 1907 that X rays and γ rays were not in fact ether waves, but rather par- ticles, a neutral pair at that: electron plus positively charged particle. Bragg’s serious research began at a late age, 41, after twenty pleasant years at the University of Adelaide, Australia, where he played golf and hobnobbed with government of- ficials. He announced his new in- tellectual work in a Presidential Address to the Australian Associa- tion for the Advancement of Science during which he made a critical review of Rutherford’s work, ques- tioning the law of exponential decrease for the absorption of α rays. For two and a half years he published a paper every few months, work that led him to make the radical state- ment that X rays were particles. His idea was based on two facts: (i) X rays excite fewer gas molecules in their path than would be expected from a wave-like disturbance, and (ii) the
Arnold Johannes Wilhelm Sommerfeld, 1868–1951. (Courtesy of the AIP Niels Bohr Library)
BEAM LINE 17
velocity of the electrons excited by X rays is greater than could be giv- en to them by a wave. By this time Bragg and his physicist son were back in England, and their theory caused great controversy even in the country where particles were in favor and where exotic modeling of physical phenomena was well tolerated. Their most vociferous opponent was Charles Barkla, who argued that the ionization of matter was a secondary effect not needing to be directly attributable to the wave-like nature of X rays. We will return later to the problem of the concentration of X- ray energy, unexplainable in terms of waves, as it bears on Louis de Broglie’s insight into the wave nature of matter.
X RAYS AS A PROBE OF THE
STRUCTURE OF MATTER
Before we turn to our final act in the almost thirty year drama to un- derstand the nature of X rays, let us turn aside to follow another direc- tion that the work on X rays took, a shift from the investigation of the nature of X rays to their use in prob- ing the structure of crystals and of atoms. That story will take us back to Roentgen and the center for phys- ics he built up at Munich. While at Würzberg, Roentgen had been agitating for an extra position in physics. He wanted a position for theoretical physics, a newly emerg- ing specialty of German origin that followed by several decades the crys- tallization of physics itself in the mid-nineteenth century. (In 1871 James Clerk Maxwell hesitated in giving his support to the creation of a Physical Society in London. He
wondered whether such a discipline distinct from chemistry existed!) When in 1899 Roentgen was of- fered a position at Munich and the chance to build up physics there, he accepted. Five years later, in negotiations with the minister of education over another possible move, this time to the Reichsanstalt, Roentgen received, in return for a pledge to stay in Munich, a second institute, for theoretical physics, to complement his existing institute for experimental physics. When Emil Cohn and Emil Weichert succes- sively declined the offer of a position, it was given to Privatdozent Som- merfeld, who joined Roentgen in Munich and shared his desire to build up physics there to the quality of the institutes in Göttingen, Berlin, and Leipzig. In the work on quantum theory of the next two decades, Munich would join Copenhagen and Göttingen as the main centers on the Continent. Sommerfeld was initially unen- thusiastic about assistant Max von Laue’s idea that regularly spaced atoms in a crystal might act as a dif- fraction grating for X rays, the fine distances between the atoms serving, as no hand- or machine-ruled grating could, to diffract ultra-high frequen- cies. If, of course, that is what one thought X rays were! Sommerfeld, pushing the impulse hypothesis, was
Radiographs of tropical fish made by J.N. Eder and E. Valenta of Vienna, Jaunary 1896 and presented to Roentgen. (Burndy Library, Dibner Institute, Cambridge, Massachusetts.)
Roentgen picture of a newborn rabbit made by J. N. Eder and E. Valenta of Vienna, 1896. (Burndy Library, Dibner Institute, Cambridge, Massachusetts.)
experimental demonstration were the British, specifically the Braggs and Henry Moseley. In view of the German results the Braggs had come to believe that X rays were of an elec- tromagnetic nature, but they insist- ed that the rays must have some sort of dual existence as they were able to concentrate their energy. But the continuing puzzle as to their nature did not stop the Braggs from recog- nizing the practicability and impor- tance of a new field of study, X-ray crystallography. The new field was pioneered by the Braggs. They were inspired by the Cambridge theorists who argued that a diffraction grating imposes a struc- ture on an inhomogeneous pulse of white light, picking out, as if in a Fourier transform, the wavelengths into which the beam can be decom- posed. William Henry Bragg and his son, William Lawrence Bragg, argued by analogy that the crystal, by dint of the distance between planes of atoms, imposes a similar structure on an inhomogeneous pulse of X rays. When the X rays are reflected off two successive planes of atoms in the crystal, they interfere construc- tively if the difference in the distance traveled is equal to an integral num- ber of wavelengths. Thus the famous Bragg condition
n λ = 2 d sin θ,
where d is the distance between planes and θ is the angle of reflection. Using an X-ray tube and a colli- mating slit to produce the incom- ing rays; using various minerals, quartz, rock salt, iron, pyrite, zincblende, and calcite, as three- dimensional diffraction gratings; and
BEAM LINE 19
using a photographic plate or an ionization chamber (depending on the strength of the incoming rays) as a detector—the Braggs proceeded with the first measurements in X-ray spectroscopy. By 1913, just a year af- ter they had pioneered the method, crystal analysis with X rays had become a standard technique. The results not only gave insight into the structure of crystals but also into the nature of the anti-cathode that produced the rays. The first person to notice that X rays can be characteristic of the sub- stance that emits them was Charles Barkla, the opponent of the Braggs in the matter of X rays as neutral par- ticles and a professor at the Univer- sity of Edinburgh who spent over forty years examining the properties of secondary X rays. Between 1906 and 1908 he had noticed that ele- ments emit secondary X rays with a penetrating power in aluminum that is distinct for each element. To distinguish between the hardness of the characteristic rays, he intro- duced the terminology K and L rays. It was for this discovery that he was awarded the Nobel Prize in 1917. (His subsequent work earned Barkla the reputation as something of a sci- entific crank.) What the Braggs no- ticed (see figure on next page) was that a pattern of multiple peaks with varying intensities was produced no matter what the crystal (shifted only by the varying distances between planes of atoms) as long as the ele- ment of the anti-cathode remained the same. In other words, the pattern was analogous to spectral lines emit- ted by gases in the optical frequen- cies. The person to explore this anal- ogy to its fullest was Henry Moseley,
Top right: Sir William Henry Bragg, 1862–1942. Lower right: Sir William Lawrence Bragg, 1890–1971. (Courtesy of the AIP Niels Bohr Library)
20 SUMMER 1995
a young researcher working in Rutherford’s Manchester laborato- ry during the time when Niels Bohr was visiting regularly. Moseley’s two grandfathers had been fellows of the Royal Society, and his father had founded a school of zoology at Oxford. Mosely himself was perhaps the only important atomic physicist to be educated at Oxford. In the fall of 1910 he came to work as a demonstrator under Rutherford, his salary being paid by a Manchester industrialist. He was assigned a research problem to which everyone knew the answer: how many β particles are emitted in the radioactive disintegration of radium B (Pb^214 ) to radium C (Bi^214 ). On find- ing the answer everyone expected, one, he proved his competency as an experimentalist. However, his next experiments would not be so cut and dried, nor would they receive the ready approval of Rutherford. Like the Braggs, and quite independent- ly of them, Moseley was stimulat- ed by the photographs of Friedrich and Knipping, and felt that Laue had misinterpreted them as evidence of five homogeneous X rays. He teamed up with Charles G. Darwin, grand- son of the famous evolutionist, and turned to, as he said, the “real mean- ing” of the German experiments. The Laue dots connoted the struc- ture of the crystal, not the structure of the incoming rays. When pre- senting his results to a Friday phys- ics colloquium which father Bragg attended, Moseley discovered the similarity in their understanding of the phenomena, and afterwards he wrote to his mother: I have been lazy for a couple of days recouping after the lecture I
*Nov. 4, 1912. Quoted in J.L. Heilbron, H.G.J. Moseley , p. 194.
gave on Friday on X rays. It was rather anxious work, as Bragg, the chief authority on the subject (Pro- fessor of Leeds) was present, and as I had to be cautious. However it proved quite successful and I managed to completely disguise my nervousness. I was talking chiefly about the new German experiments of passing rays through crystals. The men who did the work entirely failed to under- stand what it meant, and gave an explanation which was obviously wrong. After much hard work Dar- win and I found the real meaning of the experiments.*
For a time the Braggs, Moseley, and Darwin continued on the same track, even though Rutherford pre- sented difficulties which were final- ly overcome by Moseley’s persistent enthusiasm and by Bragg’s offer to Moseley of a visit to Leeds to teach him the techniques of X-ray spec- troscopy. Some of the questions they pursued were the old ones about the nature of X rays. How to reconcile the corpuscular nature of the rays with their ability to interfere? Bragg had compared this conundrum in the electromagnetic theory of X rays to the physical impossibility of a spread- ing circle of water waves, caused by the fall of a rock, to excite another rock to jump the same distance the wave-producing rock had fallen. The new questions concerned the elements. In July of 1913 Bohr paid a visit to Manchester and discussed atomic structure with Moseley, Darwin, and George Hevesey. The discussion revolved around the sim- ilarity, and possible differences, H. G. J. Moseley in Balliol-Trinity Laboratory, Oxford, circa 1910. (Courtesy of University of Oxford, Museum of the History of Science and the AIP Niels Bohr Library)
0 10
5 10 15 20 25
20 30 Degrees
Ionization Degrees
C (^1) C BA
B (^1)
B 2 A (^2) C 3? B (^3)
A (^1)
C (^2)
C (^2)
B 2 A (^2)
I
II
One of the earliest examples of X-ray spectroscopy. The Braggs made a seredipitous discovery: while studying the scattering of X rays off of crystals they noticed that a distinctive pattern of peaks appeared for each of the different anti-cathodes being used to produce the rays. What had initially started out as a study of the structure of crystals led to an investigation of the atomic structure of the anti-cathode elements. [Bragg and Bragg, PRS, 88A, 413 (1913).]
22 SUMMER 1995
frequency of X rays emitted. By 1916, the Duane-Hunt law was the best way to determine h , Planck’s con- stant, although neither Millikan nor Duane then subscribed to the view that energy came in discrete quan- tized units. Arthur Holly Compton also initially interpreted his results on X-ray scattering from electrons as a cut-off relation that was governed in this case by Planck’s constant rather than as proof of the quantum nature of radiation. Compton, who was later to run the Manhattan Project’s Met- allurgical Laboratory at Chicago during World War II, received his Ph.D. from Princeton just before the First World War for work on X-ray diffraction and scattering. After several years spent at Westinghouse Manufacturing Company working on fluorescent lamps, he spent a year at Cambridge’s Cavendish Laborato- ry where he developed a friendship with J.J. Thomson and carried out an investigation into the orderly change of X-ray frequency with scattering angle as the X rays scattered from electrons. As a new professor at Washington University in St. Louis, Compton published a mass of data on the relation between X-ray fre- quency and scattering angle taken with a Bragg crystal spectrometer. In 1922, a year after he had taken the measurements, and along with Peter Debye in Germany who had seen his results in the Bulletin of the Na- tional Research Council , Compton accepted Einstein’s light quantum and by extension the X-ray quantum. The explanation of the Compton- effect then became a simple scatter- ing of two elastic particles.
brought to a halt by the end of the World War I. The B r a g g s ’ w o r k continued after the war. The el- der Bragg revivi- fied the Royal I n s t i t u t i o n , where Sir Hum- phry Davy and Michael Faraday had made their chemical and electrical dis- coveries, by es- tablishing a re- search school for the analysis of organic crystals. This work would become central to the developing field of molecular bi- ology. During the war the elder Bragg worked for the Navy board to eval- uate inventions and to promote re- search with military applications. Like many British and U.S. scientists he eventually found himself work- ing on problems of submarine de- tection. Moseley, with his Eton pa- triotism, practically forced himself upon the Royal Engineers along with his friend Henry Tizard. Tizard sur- vived the Great War and subse- quently led the minds of British sci- entists into World War II. Moseley, however, died at Gallipoli in the bat- tle of Sari Bari. As Europe engaged itself in the Great War, interesting work on X rays began to come out of the Unit- ed States. Following Robert Mil- likan’s work on the photoelectric effect, William Duane at Harvard gave an exact law that related the en- ergy of cathode ray electrons to the
Arthur Holly Compton, 1892–1962. (Courtesy of the AIP Niels Bohr Library)
Electrical Laboratory but with no salary. Moseley began a thorough in- vestigation of Mendeleev’s table us- ing X rays, and moving from calcium to zinc and then to the rare earths, lanthanum to erbium. George Ur- bain, a Professor of Chemistry in Paris at one of the Grands Ecoles had been engaged for years in fraction- ating the elemental rare earths in competition with Carl Auer von Welsback, who performed his frac- tionations in an Austrian castle. Urbain recognized the power of Moseley’s technique and paid him a visit with precious samples of the last four rare earths, thulium, ytter- bium, lutecium, and celtium. He was astounded at how quickly Mose- ley’s X-ray spectrometer could de- termine that celtium was not his sought after new element, but was only a combination of lutecium and ytterbium! The Braggs’ work on crystals and that of Moseley’s on elements was
BEAM LINE 23
Few physicists had taken Einstein seriously when he predicted the light quantum in 1905. Bohr had pooh-poohed the idea. But by 1921 evidence was mounting and so was Einstein’s fame. The de Broglie brothers, Maurice and Louis, were two others who had learned from the studies of X rays of the dual nature of radiation, and Louis was inspired to suggest that matter too might have this dual nature. Maurice de Broglie’s interest in the quantum had been sparked by his secretaryship of the first Solvay Conference called in 1911 by chemist Walther Nernst to introduce the quantum concept to physical scientists, and he decided to investigate the energies of electrons excited by K and L frequency X rays. He found the old problem over which Bragg and Barkla had argued: X rays can concentrate their entire energy and pass it on to electrons. And like Bragg he concluded that X rays act both as waves and as particles. His younger brother, Louis, in a spirit of unification, longed to treat light and matter as equals. Both could be understood as particles following waves, he proposed. A “mobile” of light or X rays or of matter followed along behind an “onde fictive.” So the discussion of X rays had come around full circle. They were discovered in Roentgen’s laborato- ry as this newcomer to cathode rays was trying to puzzle out his coun- tryman Lenard’s challenge to the British. Lenard believed cathode rays to be ethereal. The British thought them particles. Soon X rays became the new mystery. Were they elec- tromagnetic waves or were they neutral pairs of particles? By 1913 the interference of X rays had con-
vinced most physicists that they were waves. The Braggs, not quite giving up, insisted that they had the properties of both waves and parti- cles. By 1922 the startling explana- tion by Compton of his scattering experiments—X-ray energy was concentrated into particle points— helped convince the science com- munity to take Einstein’s notion of light quanta seriously. And finally the work on X rays by the de Broglies, and the younger brother’s desire to put on an equal footing light and matter, gave Louis de Broglie the courage to suggest that even the good old electron (the cathode ray parti- cles!) partook of wave qualities.
ADDENDUM
The early history of X rays follows another path that I have not covered here. As the physicists wondered about the nature of X rays and used them to probe the structure of crys- tals and atoms, medical doctors used them to probe the human body and to diagnose and treat disease. Roent- gen by presenting an X-ray photo- graph of his wife’s hand to the Würzburg Physical and Medical Society in January of 1896 began the practice of radiology. A month later a German doctor used an X ray to diagnose sarcoma of the tibia in the
Niels Bohr, 1885–1962. (Courtesy of the AIP Niels Bohr Library
