(b. Budapest, Hungary, 5 June 1900; d. london, England, 9 February 1979)
physics, electrical engineering.
Dennis Gabor was awarded the Nobel Prize for physics in 1971, for the invention and development of the method of holography, to date the most influential product of his restlessly inventive intellect. By his own admission, Gabor lived on and for the compulsion to invent. Never the mechanic or, strictly speaking, an experimentalist, he saw his role more as that of conceiving future possibilities, both technical and social.
Gàbor Dénees, to give the Hungarian form of his name, was the eldest of three boys. His childhood was a period of intense and liberal intellectual stimulation within a culturally rich Jewish middle-class society. His father, Bertalan, was director of Hungary’s largest industrial firm, a coal mining company. His mother, Adrienne Kàlmàn, was a former actress. The family adopted the Lutheran faith in 1918, and although Gabor nominally remained true to it, religion appears to have had little influence in his life. He later acknowledged the role played by an antireligious humanist education in the development of his ideas and stated his position as being that of a “benevolent agnostic.” By the age of fifteen, he was performing home experiments with his younger brother, George, and was set on a scientific career.
School in Budapest was followed by military training toward the end of world war 1 and then a course in mechanical engineering at the city’s technical university, where he enrolled in 1918. Gabor left during the third year because of his distaste for registering for military service under a reactionary government. His formal education was completed at the Technische Hochschule in Berlin, where he received the diploma in electrical engineering in 1924 and the doctorate in 1927. His technicalingenuity found employment in industry in Germany, briefly in Hungary, and from 1934, in england.
On 8 August 1936 Gabor married Marjorie Louise Butler, a fellow employee of the British Thomson-Houston Company; they had no children. From 1949 until his retirement in 1967, he worked at Imperial College, London. By the latter year, his Interests were dominated by his concern for the future of industrial societies. Which found expression through his involvement in the Club of Rome, of which he was a founding and very active member. Elected a fellow of the Royal Society in 1956, he was also the first émigré to be elected an honorary member of the Hungarian Academy of Sciences in 1964.
Short and sturdy, Gabor enjoyed good health for most of his adult life, apart from a serious attack of thrombosis-phlebitis in the legs in 1961. In 1974, however, he suffered a severe cerebral hemorrhage, following which he could neither read nor write.
Besides his work on hologrophy, gabor’s scientific and technical contributions include developments in the theory of communications, plasma theory, and technical ideas embodied in more than a hundred commercial patents. Like anglers, however, inventors are haunted by the ones that get away. Much of Gabor’s scientific work was influenced by the fact that he had not invented the electron microscope.
In 1931 Ernst Ruska and Max Knoll developed the first two-stage electron microscope. Within a few years Ruska had improved “this wonderful instrument,” as Gabor more than once described it, to an extent that its resolving power improved on optical instruments. Gabor could never forgive himself for not having done it himself and spent years trying to make up for What he considered his missed opportunity. The cause of this deep regret—indeed, envy—lay in the possibilities presented by his own doctoral research at the Technische Hochschule in Berlin, carried out between 1924 and 1926, on the measurement of fast surges in high-voltage power lines. These surges, caused by lightning, often resulted in considerable damage.
In order to record these surges, Gabor developed a fast-response cathode-ray oscilloscope of advanced design. Particularly significant was the replacement of the conventional long focusing solenoid by a short coil, encased in iron, intended to confine the field within the coil and prevent the intrusion of stray magnetic fields. Without realizing it, Gabor had constructed in crude form the first iron-shrouded magnetic electron lens, the forerunner of the modern high-resolution magnetic lens of an electron microscope.
The full significance and correct explanation of the working of the magnetic electron lens were soon grasped by Hans Busch (1926). After this, the opportunity was clear. As Gabor himself later commented: “How could anybody capable of putting two and two together not think of an electron microscope?” He recalled suggesting in 1928 to his compatriot and friend Leo Szilard that he had the expertise within his grasp to build such a Microscope. His cathode ray oscilloscope provided, in a crude embryonic form, the basic technology. To his lasting regret, he made no move. The completion of his doctorate had made him “temporarily allergic” to electrons, he later explained. In addition, the usefulness of an electron microscope was not immediately clear.
After completing his doctoral dissertation, Gabor found work in the physics laboratory of Siemens and Halske at Siemensstadt, where he studied lamp technology. His interest in the improvement of mercury vapor lamps had developed from his private research aimed at detecting the “mitogenic rays” claimed to be emitted from growing onion roots. The lamps were used in these experiments to attempt to induce mitosis. He remained at Siemens until 1933. Within weeks of Hitler’s rise to power, Gabor’s contract with the company was terminated. Returning briefly to Hungary, he used the patents of a new lamp design to negotiate an inventor’s agreement to go to England and work on its improvement. Though never a success, his plasma lampgave him a foothold in the British Thomson-Houston Company at Rugby in Warwickshire. With the end of the agreement in 1937, he was appointed to the permanent staff, where he remained until 1948. Gabor was the only refugee to be employed by the B.T.H. Research Laboratory. His vivid personality enriched the establishment, and his scientific output remained considerable. Nevertheless, Gabor found this period in his life, during which he made acclaimed contributions to the theory of communications, and at the end of which he established the principles of holography, sterile and at times depressing.
His time in Berlin had been one of the great periods of Gabor’s life. He had gone to the university as often as possible to witness at first hand the work of Albert Einstein, Max Planck, Walter Nernst, and Max von Laue. He moved in a circle of expatriate Hungarians that included John von Neumann, Eugene Wigner, Leo Szilard, and Michael Polanyi, as well as his close friend Peter Goldmark. In contrast, from 1939 until 1945 Gabor was isolated from the mainly classified work at Rugby, accommodated in a hut built for him on the fringe of the restricted area that could be reached only by a specified route. He was virtually cut off from the scientific literature, Nature being the only journal he could obtain regularly. In 1942 his father, who had been a great influence on his life, died in Hungary. About 1943 Gabor began to give more serious thought to the electron microscope.
It is clear that the electron microscope had never been far from Gabor’s mind from the moment he realized what he had missed in its initial development. Now he had an opportunity to make what he thought of as a comeback in the field. Thereal prize, as he saw it, would be gained for producing an instrument that could “see” individual atoms. Yet despite gradual improvements in the resolving power of electron microscopes, a theoretical barrier set by a compromise between diffraction effects at the aperture edge and spherical aberration placed crucial practical limits short of the resolution needed to focus atomic lattices. After Otto Scherzer first pointed out the limitation posed by spherical aberration in 1936, several authors attempted to calculate the limit to resolving power set by various imperfections, and in 1943 the exact rule was elaborated by Walter Glaser.
For Gabor this provided just the kind of challenge his inventive mind needed. The barrier to progress was of a technical nature, yet formidable. At the time, his main work, on developing his ideas on infrared detection, was going poorly. His initial efforts at thinking of ways around the theoretical limits placed on the electron microscope brought no ready solutions. His attempts to interest others in the problem were unsuccessful. In 1946 the theoretical limit was virtually reached in practice by James Hillier and Edward Ramberg. By then Gabor was seriously considering offers from the United States, frustrated by what he felt to be a lack of support for his ideas at B.T.H., in particular for his latest project, to develop a 3-D projection system. A number of factors conspired to keep Gabor in England. In 1947 he became a British citizen. In the same year he made “my luckiest find yet,” the one he hoped would enable him to get his own back on the electron microscope, and so reveal the atomic lattice. around the barrier of the theoretical limit to resolution. During the Easter holiday in 1947, he became a British citizen. In the same year he made “my luckiest find yet,” the one he hoped would enable him to get his own back on the electron microscope, and so reveal the atomic lattice.
Gabor had been searching for the “trick” to get around the barrier of the theoretical limit to resolution. During the Easter holiday in 1947, he was sitting on a bench at the local tennis club when an idea suddenly came to him: Why not take an electron picture distorted by lens imperfections and correct it by optical means? A few calculations convinced him he was right.
Gabor was proposing a two-stage process. In the first stage an interference pattern produced by the interaction of electrons diffracted by the object and a separate but coherent reference beam of electrons would be photographically recorded on transparent film. Gabor argued that this interference pattern, or “hologram,” as he later called it, would carry the complete information needed to reconstruct an image of the object, using an optical system free from the limitations of electron optics. In the second stage, the hologram would be scaled up by a factor in the ratio of the wavelength of the light used in the reconstruction to the wavelength of the electron beam. Thenew hologram would then be illuminated with a light wave of the same aberration as the electron wave to, in theory, reveal an exact replica of the original object, magnified by the scaling factor.
In July 1947 Gabor, assisted by Ivor Williams, began experiments at B.T.H. to establish the prin ciple by using a purely optical model—that is, using visible light instead of electrons—with a mercu ry vapor lamp as a source of coherent light, to pro duce the interference photographs of simple two dimensional images. Five months later he was able to show his close confidant Lawrence Bragg his first successful wavefront reconstructions: hazy im ages of simple printed words used as objects. Even when Bragg fully understood the theory, he still stated that it was a miracle it should work. The first public indication of Gabor’s success came with a preliminary note to Nature, published on 15 May 1948. The following year he wrote a more complete theoretical treatment, for the Proceedings of the Royal Society, in which he introduced the word “hologram” and indicated possible applications in light optics. Among these was the ability, using the same method, to record the data associated with 3-D objects in one interference photograph.
Gabor’s private correspondence was full of enthusiasm. To Max Born, he declared that his holographic reconstructions had made him happier than anything he had done in the last twenty years. And he told Arthur Koestler: “I missed inventing the electron microscope when I was 27, and I have every intention to make a comeback, with a fresh start, at 48.” For the moment he was very happy. Within a few years he again felt the inventor’s frus tration as everything seemed to turn sour.
In his Nobel Lecture, Gabor acknowledged the influence of both Lawrence Bragg and Frits Zernike on his ideas in wavefront reconstruction, as he called his new principle. The idea of a two-step imaging process came directly from Bragg’s X-ray micro scope, described in 1942, in which holes were drilled in a brass plate that corresponded to the photo graphically recorded image of an X-ray diffraction pattern produced by a crystal lattice. When the plate was illuminated with monochromatic light, an image of the crystal structure could be viewed through the microscope. The double diffraction pro cess explicit in the X-ray microscope is crucial to holography.
At the time, neither Gabor nor Bragg was aware that the method had been suggested by the work of Mieczislav Wolfke in 1920. Bragg’s method was limited, however, to cases where both amplitude and phase of the wave were preserved. In 1950 Martin Buerger extended the principle to crystals producing known phase changes by using “phase shifters.” Gabor’s crucial addition to the double diffraction concept was the preservation of phase information through the introduction of the coherent reference wave. The whole information (hence “hologram,” from the Greek holos, meaning “whole”) would thus be preserved in the record ed interference pattern, allowing a complete re construction.
A coherent background wave had been used with great success by Zernicke for his investigation of lens aberration by phase contrast. To this Gabor added the concept of reconstruction. The electron shadow microscope of Hans Boersch (1939) was similar to the first stage of Gabor’s process, except in the use of coherent illumination. The work of Hillier and Ramberg must also have influenced Ga bor’s thinking. In early 1947 they published work showing that electrons, penetrating through mem branes, could interfere with the illuminating wave passing the edge, Gabor had advance notice of these results and had suggested adapting the concept of Zernicke’s phase-contrast microscope for use with electrons. He was a strong believer in the rational cumulative development of knowledge and was generous to those he recognized as having influenced his work. He had stood on the shoulders of Bragg and Zernicke, he claimed in his Nobel Lecture.
Early in 1949 Gabor moved to an academic ap pointment at Imperial College of Science and Tech nology, London, as reader in electronics, although at least until 1952 he was still debating whether to accept offers of a permanent move to the United States. Final acceptance that his marriage would be childless probably clinched the issue. At the same time, his new post relieved him of some of the frustrations he had felt at B.T.H., as a succession of postgraduate students allowed him to put many of his ideas into practice. Together they built a Wilson cloud chamber, an analog computer, a flat television tube, and many other devices.
Meanwhile, attempts were under way to build a working electron microscope, based on Gabor’s holographic principle, in the new research labora tories of Associated Electrical Industries, at Al dermaston. A.E.I. was the parent company of both B.T.H. and Metropolitan-Vickers, the latter a pioneer in the development and manufacture of electron microscopes. In 1950, with government funding, this program of holographic electron microscopy got under way with the participation of Michael Haine, James Dyson, and Tom Mulvey. Gabor re mained closely involved as a consultant. By 1953 they had demonstrated that reconstituted images were possible with use of the technique, although overall results emphasized the limitations. They were unable to reduce the resolution to the point that there was any real advantage over more traditional methods. The work was held in abeyance from 1953 until a decision was made to close it down in 1955.
Gabor took little consolation in the modest im provements made and let his disappointment be known. With feelings running high, he declared to Haine: “… it was a very ill wind which I let out now almost eight years ago which blew nobody any good, least of all to myself.” The holographic prin ciple, on which Gabor had set such high hopes, appeared to be receding to the status of a scientific curiosity.
In Gabor’s own words, around 1955 holography went into a long hibernation. Among other early attempts to apply the technique, Gabor, with Walter P. Goss, constructed a holographic interference mi croscope that failed to arouse any interest from the optical industry. In California, Hussein El-Sum, Paul Kirkpatrick, and Alberto Baez attempted to prodace X-ray holograms, and Gordon Rogers in Britain worked with radio waves. In general, however, interest in the subject seemed to have petered out. Far from Gabor’s intended use of holography, only the recognition that it could be successfully applied to radar stimulated the research that kept an interest alive.
Doing classified radar research at the Willow Run Laboratory of the University of Michigan, Emmett N. Leith first became aware of Gabor’s work in late 1956, although not until 1960 did he and his collaborator, Juris Upatnieks, initiate a research program. In that year they duplicated Gabor’s optical experiments. Using simple means, they succeeded in overcoming one of the major limitations expe rienced by Gabor and others, the elimination of a spoiling “twin” image produced in the reconstruc tion. In 1962 they produced the first laser holograms.
The availability of the powerful coherence pro vided by lasers is often seen as the key factor in the revival of holography. This is only partly true. The invention of the laser coincided with a clear increase of interest in holography, and many of the achievements of this period could have been, and in some cases were, produced without the use of lasers. It is true, however, that the dramatic three dimensional holographic images could be produced only by diffuse reflection using lasers.
In 1963 and later, Gabor claimed he had the com plete idea for a laser back in 1950, but could not find a student willing to take up the project. Optical holography, meanwhile, was enough to make Gabor a celebrity and truly give him his “comeback,” even if it was not exactly in the way he intended. He experienced the inventor’s dream of seeing an idea grow into concrete applications. These came in abundance, and many further applications of the general principles of wavefront reconstruction are still possible in the future.
Concern for the future came to dominate Gabor’s later years. In the same year that Leith and Upatnieks revealed their laser holograms. Gabor published the book he had written in “a year of Saturdays.” In venting the future, in which he eloquently ex pounded his concern for the future of industrial civilization, faced with the triple threat of over population, nuclear weapons, and the “leisure so ciety.” It expanded on themes first developed in his inaugural address following election to a personal chair of applied electron optics at Imperial College in 1958 (published in an abridged form in Encounter in 1960). His major concern was his perception of a mismatch between technology and social insti tutions, and the necessity for inventive people to turn to “social inventions” as a first priority—indeed, this became his own priority. He promoted the mo bilization of “a force of thinkers” to provide visions thirty or forty years ahead and found opportunity for such a project when, in 1968, he became a founding member and active participant in the Club of Rome, an elite drawn from diverse backgrounds who were interested in the study of global problems. His views are reflected in Inventing the Future and its sequel. The Mature Society. In the former he confesses, “Any book on the future will tell more of its author than about things to come.” Gabor’s beliefs were essentially those of a conservative and benevolent humanist. Although his books have an optimistic outlook, in private he was a pessimist and held a sneaking longing for the past. An avid reader of science fiction, he also, by contrast, ad mired the more conservative writings of Evelyn Waugh. One of the greatest influences on him, however, came from Aldous Huxley. Haunted by the Malthusian threat of overpopulation, Gabor shared many of the views of the British eugenicists. Writings on the subject by Julian Huxley and Charles Galton Darwin also influenced him. Politics and religion had little part in Gabor’s view of things. His pes simism came more from what he saw as the “ir rationality” of human behavior. He opposed the Vietnam war and saw the space program as “the last collective folly of mankind.” At the same time, and in contrast, he viewed the student unrest of the late 1960’s as a symptom of social malaise and the decline of morality.
In his later years, Gabor still found time for sci entific interests. With the revival of holography, he was much in demand as a speaker on the subject. He also made further important contributions of his own, including introduction of the application of holography to computer data processing. After re tiring he remained a research fellow and professor emeritus of Imperial College. Much of his time, however, was divided between the CBS Laboratories in the United States, where he did much of his later work as a part-time consultant, and a summer home in Italy.
In December 1971 came the Nobel Prize. Gabor had made his comeback. Yet by the time of his Nobel Lecture, he had virtually given up any idea of the realization of his original aim, holographic electron microscopy. One year later, Lawrence Bartell, at the University of Michigan, realized how it would be possible to devise a holographic electron clouds in gas-phase atoms. several undergraduate students developed the method to a point where atomic and molecular photographic images of a quality allowing bond lengths to be measured with a ruler were produced. In April 1974, Bartell informed Gabor of these developments, which immediately set him to designing his own holographic electron microscope.
During the summer of that year, Gabor suffered a severe stroke, after which he could neither read nor write, and later almost totally lost the power of speech, although his intellect remained unim paired. Four years later, after a summer spent at his Italian home, he was confined to his bed, and, during the following winter, he died peacefully in a London nursing home. In 1977 he had visited the newly created Museum of Holography in New York City.
Gabor’s inventions may yet hold greater future significance. The basic optical hologram, however, will remain one of those things that stir the imag ination. As Emmett Leith remembered about du plicating Gabor’s original optical experiments: “The results were only what we had expected, yet the physical realization was rather awesome.”
I. Original Works. A complete list of Gabor’s publications and references to many of his patents are in Allibone (see below). His works include Inventing the Future (London, 1963); “Holography 1948–71,” in Le Prix Nobelen 1971 (Stockholm, 1972), 169–201; The Ma ture Society (London, 1972); and “The Principle of Wavefront Construction. 1948.” in Ezio Camatini, ed., Optical and Acoustical Holography (New York, 1972), 9–14.
Gabor’s private papers and correspondence are in the archives of Imperial College of Science and Technology, South Kensington, London. Autobiographical notes are at the Royal Society of London.
II. Secondary Literature. T.E. Allibone, “Dennis Gabor,” in Biographical Memoirs of Fellows of the Royal Society, 26 (1980), 107–147, the most comprehensive out line of Gabor’s life and work; Michael Edward Haine and Vernon Ellis Cosslett, The Electron Microscope London, 1961; repr. 1962); Emmett N. Leith, “Dennis Gabor, Holography and the Nobel Prize,” in Proceedings of the IEEE, 60 (1972), 653–654, and “The Legacy of Dennis Gabor,” in Optical engineering, 19 (1980), 633–635; and Tom Mulvey, “Fifth Years of High Resolution Electron Microscopy,” in Physics bulletin, 34 (1983), 274–278.)
S. T. Keith
"Gabor, Dennis." Complete Dictionary of Scientific Biography. . Encyclopedia.com. (February 25, 2018). http://www.encyclopedia.com/science/dictionaries-thesauruses-pictures-and-press-releases/gabor-dennis
"Gabor, Dennis." Complete Dictionary of Scientific Biography. . Retrieved February 25, 2018 from Encyclopedia.com: http://www.encyclopedia.com/science/dictionaries-thesauruses-pictures-and-press-releases/gabor-dennis
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The Hungarian-British physicist Dennis Gabor (1900-1979) received the Nobel Prize in Physics in 1971 for his invention of holographic photography.
Dennis Gabor was born on June 5, 1900, in Budapest, Hungary, to S. Berthold and Ady (Jacobovits) Gabor. The son of a businessman, he received his education at the technical universities of Budapest (1918-1920) and Berlin (1920-1927). He earned both his diploma and his doctorate in engineering from the Technische Hochschule, in Charlottenburg, Germany, the former in 1924, the latter in 1927. He remained in Berlin upon graduation, working as a research engineer for Siemens and Halske until Hitler's rise to power in 1933. At that point he left Germany for Britain, taking a job with the British Thomson-Houston Company in Rugby, England.
Gabor stayed with the British firm for 15 years, from 1933 to 1948, working on the improvement of the resolving power of the electron microscope. The electron microscope had increased resolving power a hundredfold over the finest light microscopes, yet it still fell short of allowing scientists to "see" atomic lattices (the patterned arrangement of atoms, not individual atoms, which are too small). The image was distorted in two ways—fuzziness (as if one's camera were out of focus) and sphericity (as though one were looking through a raindrop). If one improved the former, the latter worsened, and vice-versa.
In 1947 a brilliant solution occurred to Gabor. What if one were to use the diffraction pattern (the fuzziness) in a way which provided one with all the information about the atomic lattice. That is, why not take an unclear electron picture, then clarify that picture by optical means. This was the genesis of holography. Gabor proposed to take an electron beam of light and split it in two, sending one beam to an object, the other to a mirror. Both would initially have the same wavelength and be in phase (coherent), but upon reflection from the object and the mirror back to the photographic plate, interference would be set up. Imagine ocean waves rolling in upon a long, sandy beach, one following another. Imagine them all equal in size, intensity, and timing. Now imagine you could split the beach in two, with two sets of waves coming in upon two different beaches. Tilt these two at an angle of your own choosing, superimpose them, and imagine the interference the waves would create for each other. This interference would not be completely chaotic, but would actually follow a pattern. From this "diffraction" pattern, one could reconstruct the initial waves. Now vary these initial waves in size, intensity, and timing (which might be imagined as due to different weather conditions out at sea). The diffraction pattern would differ correspondingly, and even the weather conditions might be hypothetically reconstructed. This is what Gabor wished to do with electron beams. The beam from the mirror would be unchanged, but the beam reflected from the object would contain all the irregularities imposed upon it by that object. Upon their meeting at the photographic plate, the two beams would be generally incoherent, and an interference pattern would occur. This interference could then be captured upon film, and if light were then shone through this film, it would take on the interference pattern and produce an image capable of three-dimensional reconstruction.
Gabor worked out the basic technique by using conventional, filtered light sources, not electron beams. The mercury lamp and pinhole were utilized to form the first, imprecise holograms. But because even this light was too diffuse, holography did not become commercially feasible until 1960, with the development of the laser, which amplifies the intensity of light waves. Nevertheless, Gabor demonstrated mathematically that holography would work even with electron beams—just as his experiments showed it worked with ordinary light. The major practical problem remaining with the electron microscope prior to 1960, however, was not left unchallenged by Gabor—this was the double image incidentally obtained by the holographic process. Gabor was able to use the very defect of electron lenses—spherical aberration—to remove the second image.
Gabor published the principle of holography and the results of his experiments in Nature (1948), Proceedings of the Royal Society (1949), and Proceedings of the Physical Society (1951). This work earned him in 1948 a position on the staff of the Imperial College of Science and Technology, London. In 1958 he was promoted to professor of applied electron physics, and he held that post until his retirement in 1967. His other work consisted of research on high-speed oscilloscopes, communication theory, physical optics, and television, and he was awarded more than 100 patents. Yet Gabor was not the pure scientist or isolated inventor; many of his popular works addressed the social implications of technological advance, and he remained suspicious of assumptions of inevitable technological progress, nothing the social problems it could not solve as well as the ones it caused.
Gabor received many honors. In 1956 he was nominated to the Royal Society; he was made an honorary member of the Hungarian Academy of Scientists; and in 1971 he received the Nobel Physics Prize for his holographic work. He died in London on February 8, 1979.
There is little biographical information on Gabor, though some can be gleaned from Who's Who in Science: Antiquity to Present, edited by Allen G. Debus (1968) and other recent dictionaries of scientific biography. His own explanation— historical and scientific—of holography can be found in his Nobel Lecture (1971), contained in Les Prix Nobel (Stockholm, 1972). This French title nevertheless contains his English lecture, and a holographic photoplate is enclosed to further illuminate the subject. Of his popular works, Innovations: Scientific, Technological, and Social (1970), The Mature Society (1972), and Proper Priorities of Science and Technology (1972) are readable. They are also repetitive, and one would do well to choose only one of them, perhaps the most recent. □
"Dennis Gabor." Encyclopedia of World Biography. . Encyclopedia.com. (February 25, 2018). http://www.encyclopedia.com/history/encyclopedias-almanacs-transcripts-and-maps/dennis-gabor
"Dennis Gabor." Encyclopedia of World Biography. . Retrieved February 25, 2018 from Encyclopedia.com: http://www.encyclopedia.com/history/encyclopedias-almanacs-transcripts-and-maps/dennis-gabor
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"Gabor, Dennis." World Encyclopedia. . Encyclopedia.com. (February 25, 2018). http://www.encyclopedia.com/environment/encyclopedias-almanacs-transcripts-and-maps/gabor-dennis
"Gabor, Dennis." World Encyclopedia. . Retrieved February 25, 2018 from Encyclopedia.com: http://www.encyclopedia.com/environment/encyclopedias-almanacs-transcripts-and-maps/gabor-dennis