Robert W. Wood: The Scientist who Played with Optics

Barry R. Masters

Robert Williams Wood viewed the natural world as his playground. For him, science was a highly creative endeavor to be approached with curiosity and awe. Wood was also a scientific maverick who questioned all dogmas. Above all, he was an experimentalist: He tested conclusions, built his own apparatuses, and was excited about every part of the process.



figureWood at Johns Hopkins University, with his mosaic replica diffraction grating.

Robert W. Wood was born in May of 1868 in Concord, Mass., U.S.A. He attended the Roxbury Latin Academy with the intention of becoming a priest. However, that plan didn’t last long, as he soon became fascinated with the natural world.

By the time he attended Harvard, between 1887 and 1891, he knew he wanted to pursue a scientific life. He was a creative and highly independent thinker, and his personality often clashed with the rigidity of his professors. From an early age, Wood wasn’t afraid to question the establishment. For example, when Wood’s geology professor and mentor Nathaniel Shaler described his glacier theory—in which Shaler posited that high pressure at the bottom of a glacier would convert the ice to water—Wood didn’t hesitate to construct an apparatus to test Shaler’s conclusion. Wood’s device was made from a block of aluminum and a tightly fitted moveable aluminum cylinder. Inside the bore, Wood placed snow that contained a lead bullet that had been suspended off the bottom.

Wood put the device in subfreezing temperature and then subjected the cylinder to pressures that greatly exceeded those at the bottom of a glacier. If the ice formed water, as Shaler had predicted, the bullet would fall to the bottom of the bore. However, when Wood disassembled the frozen apparatus, the bullet remained suspended in the ice above the bottom of the bore, thus debunking Shaler’s theory. Wood published the result in the American Journal of Science.

Wood also wasn’t afraid to delve into unconventional areas. After conferring with Prof. William James—a pioneering psychologist—Wood decided to test the psychological effects of cannabis indica—a plant containing the same psychoactive molecule as marijuana. He wrote his personal account of the experience, which was published in the New York Herald on September 23, 1888, and excerpted in James’ Varieties of Religious Experience.

Wood graduated from Harvard with an honorable mention in chemistry and natural history.

Grad school and early career

Wood’s next stop was Johns Hopkins, where he began his studies for a doctorate in chemistry. There, he frequently performed spectroscopic experiments in the laboratory of Henry Rowland. In 1892, following his father’s death, Wood decided to leave Hopkins and get married. He then applied for a position at the newly founded University of Chicago. Wood’s job was to clean the apparatus following the lectures and demonstrations of Prof. Henry Stokes. Most important, the position gave him access to the chemical laboratories and the library.

Within two years, Wood had completed enough research to qualify for his Ph.D. in chemistry—or so he thought. Just as he was finishing, he was told that he would need to meet new graduation requirements, which included a number of exams in math and physics. Wood objected to the new rules but did not succeed in convincing the university president to change them. He left the university without a doctorate.

In 1894, Wood left for Germany to work in Leipzig with Wilhelm Oswald, the world’s most eminent physical chemist. He also worked at the University of Berlin (1894-1896). When he and his family returned to the United States, Wood, now 29, obtained a position as a junior instructor at the University of Wisconsin in Madison.

When asked to lecture on physical optics—a subject that Wood had never studied—he dove into a year-long self-study program. At the end of this period, he realized that the standard textbook, Thomas Preston’s Theory of Light, did not cover the modern advances of the subject, so Wood decided to write his own book. Physical Optics took five years to complete. It became a seminal volume in optics that was translated into German, French, Russian and other languages. OSA reprinted the third edition in 1988.

Throughout the history of science, we see numerous instances in which the invention of an instrument facilitates new discoveries. The spectroscope is a prime example. In 1859, Robert Bunsen and Gustav Kirchhoff discovered that the light from substances heated until they are luminous and passed into a spectroscope shows a characteristic spectrum (intensity versus wavelength or color of the light) that can be used to identify each substance. The spectroscope was instrumental in studies of the nature of matter throughout the universe—from atoms to planets to galaxies.

Wood began his work on the optical properties of sodium vapor and continued this line of research throughout his career. He also investigated the spectroscopy of iodine and mercury. (Niels Bohr cited his work on sodium vapor in his first paper on the structure of atoms.) In addition, Wood initiated studies on the nature of diffraction gratings. While at the University of Wisconsin, he invented a new technique of color photography.

A productive professor

After Henry Rowland died in 1901, Wood was offered the position of full professor in experimental physics at Johns Hopkins. Using a ruling engine that accurately cut sets of parallel grooves into a metal plate, Rowland had ruled the world’s best diffraction gratings at Hopkins. When Wood joined the faculty, he further improved upon the construction of Rowland’s ruling engine, resulting in the fabrication of higher resolution diffraction gratings for spectroscopy.

Wood taught three lectures per week on physical optics and thus was able to concentrate on research. He initiated a series of investigations with ultraviolet light photography. He photographed scenes as widely divergent as lunar landscapes and terrestrial vegetation.

Wood was not only inventive and independent; he was also extremely productive. For example, in order to advance his studies of sodium vapor, he procured a $500 grant from the Carnegie Institute and $1,000 from his mother to construct a Michelson interferometer. With this instrument, he rapidly gained international recognition. In the words of a German physicist, “Wood produces like a rabbit.” Later, when OSA recognized Wood with the Ives Medal, the citation included the definition of a “Wood experiment” as that which is distinguished by unusual ingenuity and efficacy, especially if done by simple means.


figureWood viewing his rotating mercury mirror in rotation. The distortion is astigmatism due to the oblique incidence.

Discovery of resonance radiation

One of Wood’s first discoveries was an apparent exception to Stokes’s law of fluorescence, which states that the fluorescent emission always occurred at a wavelength longer than the excitation wavelength. Wood observed that, in sodium or iodine vapors, the fluorescence wavelength coincided with the excitation wavelength; he named the phenomenon resonance radiation, which is defined as the re-emission of absorbed radiation without a change of wavelength.

Wood was a clear communicator who would enthusiastically demonstrate his experimental findings to anyone interested. During a trip to Cambridge to attend a meeting of the British Association for the Advancement of Science, Lord Rayleigh invited him to visit his Terling estate and laboratory. Wood was thrilled, as he held Rayleigh in high regard and considered his collected works as his personal Bible.

Wood arrived at Terling with a suitcase that contained his demonstration apparatus of the resonance radiation of sodium vapor as well as his diffraction apparatus for making color photography: glass tubes and bulbs, rubber tubing, various prisms, lenses and a gas burner to vaporize the sodium. During the visit, Wood demonstrated his key findings on resonance radiation. He also discussed his results with H. Kayser of Bonn—the leading spectroscopist in Germany—and Otto Lummer, a famous physicist from Breslau University.

Upon his return to Hopkins, Wood moved his spectroscopic apparatus to a room in the astronomy tower that contained the Johns Hopkins astronomical telescope. Lacking sufficient intensity in his electric arc light source, Wood thought he would try sunlight. The sun has been used as a source in many developments in physics and medicine: Newton spectrally dispersed sunlight with a prism; Raman discovered his effect with a heliostat; and the German ophthalmologist Gerhard Meyer-Schwickerath used the sun as the light source when developing the first retinal photocoagulators.

In his new laboratory venue, Wood designed and constructed a spectrograph; it had three large prisms of flint glass and large achromatic lenses. He mounted a heliostat on the window sill and focused the sunlight on the entrance slit of a monochromator. The emerging light from the exit slit was then focused on the sodium vapor in the quartz tube, and the fluorescence from the sodium vapor was directed onto the entrance slit of his newly constructed monochromator with a mirror and condensing lens.

Wood could then alter the excitation light by turning the prisms in the excitation monochromator. He could either visually observe the spectrum of the sodium vapor or capture it on a photographic plate attached to the spectrograph. With this apparatus, he would make a major discovery: He could detect and measure various groups of widely separated lines in a complex spectra that consisted of thousands of closely spaced lines.

René Blondlot and N-rays

Wood gained a reputation as a skeptic and debunker of theories that did not hold up to close scientific scrutiny. Perhaps the best example is his work to disprove Blondlot’s theory of so-called N-ray radiation. That story began in 1903, when the French physicist René Blondlot was investigating whether X-rays were particles or electromagnetic waves. Blondlot was the chair of the department of physics at the University of Nancy, a noted expert on electromagnetic radiation, and a member of the French Academy.

Blondlot knew that electromagnetic waves could be polarized, and he planned to use this characteristic to determine the nature of X-rays. He used a spark between two parallel wires as the detector; at the proper orientation of the detector, the polarized electromagnetic waves should increase the intensity of the sparks. Blondlot observed that a quartz prism could bend (refract) the electromagnetic waves that interacted with the spark detector. From the observed properties of the radiation, which differed from those of X-rays, Blondlot reasoned that, if the rays were not X-rays, they must constitute a new type of ray. He named them N-rays after the University of Nancy.

Blondlot described his experiments in a series of papers published in the French Academy’s Comptes rendus. He alleged that many metals spontaneously emit N-rays and that they could be detected with a very weakly illuminated piece of paper. N-rays could supposedly be transmitted through metal, wood and paper, but not water. They were readily transmitted through materials that are opaque to visible light; however, water and rock salt were opaque to them. Blondlot claimed that there were many sources of these N-rays: the Nernst glower (used in home lighting at that time), heated pieces of silver and sheet iron. The Bunsen burner did not produce N-rays. Blondlot claimed that the sun was a source of N-rays as well.

Subsequently, many Frenchmen claimed priority in the discovery of N-rays. But Blondlot prevailed, in spite of skepticism from many of the world’s scientists due to their inability to reproduce the reports. In 1904, the French Academy awarded Blondlot the Prix Leconte; it was given to him for his entire corpus of research; the discovery of N-rays was only mentioned at the end of the list of his achievements.

Within a year of the purported discovery, 12 N-ray papers appeared in Comptes rendus. Shortly afterwards, Blondlot reported that he had constructed a spectrometer with aluminum lenses and an aluminum prism. He said that N-rays showed dispersion (they are composed of various wavelengths) and that he had measured their wavelengths.

Jean Becqueral, the son of Henri, asserted the N-rays could be transmitted over wires. Soon many biologists, psychologists and others made exciting claims: nerves in the spinal cord emitted N-rays that could be used to detect disease. A fellow faculty member, Augustin Charpentier, stated that N-rays were emitted in human nerves and muscles and even in the human body after death. He reported an increase in N-ray intensity with motor activity and proposed a new approach for cardiac imaging. Others detected N-rays in plants. In the first six months of 1904, Comptes rendus published more than 100 papers on the new rays. Twenty French scientists claimed that they had confirmed their existence.

Meanwhile, others scientists, including Wood, could not detect the new form of radiation. Wood, who was a foreign member of the Royal Society of London, traveled to Nancy with the purpose of validating Blondlot’s claims. Wood was shown a series of experiments intended to convince him of the reality of N-rays. First, Blondlot shared with him photographs that purported to demonstrate their existence. He observed that the source of light in the experiments, a spark gap, has a variation of intensity of about 25 percent, and that source intensity variation precluded accurate measurements.

Next, Wood was shown the experiment that supposedly demonstrated the deviation of the N-rays by an aluminum prism. When the lights were turned down low, Wood secretly lifted the aluminum prism from the spectroscope, and Blondlot continued to measure the spectrum of the N-rays, noting no change in the dispersion. Before the lights were turned on again, Wood carefully replaced the aluminum prism in the spectroscope.

The next morning, Wood submitted a paper to Nature (London), in which he summarized his conclusions on N-rays. Without naming Blondlot, he stated that he visited the laboratory in which most of the N-ray experiments were carried out and that he had removed the aluminum prism from the spectrometer. He stated that this act “did not seem to interfere in any way with the location of the maxima and minima in the deviated ray bundle,” and concluded “that all the changes in the luminosity or distinctness of sparks and phosphorescent screens (which furnish the only evidence of n-rays) are purely imaginary.” Finally, Wood proposed experiments that could be used to settle the issue beyond doubt; they were never performed.

Wood’s report was published in Nature on September 29, 1904. The fallout was severe, especially for Blondlot. Subsequent to Wood’s failed verification, only two papers on N-rays were published in Comptes rendus.

The French journal Revué Scientifique proposed a blinded experiment to Blondlot. They offered to send him two boxes that were sealed and identical in every way, with two exceptions; one contained a piece of tempered steel, while the other included a piece of lead. Blondlot’s task was to determine which box emitted the N-rays. He refused to take part. Following the N-ray debacle, Blondlot continued to work at the University of Nancy until his early retirement in 1910. He died in 1930; no one has reported on the existence of N-rays since then.

In 1980, Irving M. Klotz, a professor of chemistry and biochemistry, molecular biology and cell biology at Northwestern University, wrote an article in Scientific American, in which he claimed that Blondlot’s discovery of N-rays was a mistake, not a hoax. I suggest that the reader peruse the articles of both Wood and Klotz (which are cited in the references) and decide for themselves. (If readers are still perplexed about how the N-ray affair could occur, I suggest that they Google the terms “polywater” and “cold fusion.”)

An honored inventor and writer

Wood was a creative experimental physicist and author. Aside from his remarkable corpus of spectroscopic studies on resonance radiation, he produced photographs in both infrared and ultraviolet light. He invented a filter that transmitted ultraviolet light but was opaque to visible light. It came to be known as Wood’s filter. He also developed Wood’s lamp, a useful source of ultraviolet light that is used in clinical dermatology and analytical chemistry and geology.

In addition to his masterpiece, Physical Optics, Wood and Arthur Train co-authored a science fiction book called The Man who Rocked the Earth. Wood showed his humorous side in a book of nonsense verse that he also illustrated: How to Tell the Birds from the Flowers and Other Woodcuts. Wood continually found joy and playfulness within science.

Wood was nominated for the Nobel Prize in Physics with C.V. Raman in 1930. He received honorary degrees from Berlin University, Clark University, the University of Birmingham and Edinburgh University. He received the Rumford Medal of the Royal Society, and a crater on the far side of the moon is named after him. He was a member of American National Academy of Science, the Physical Society, the London Physical Society, the Royal Swedish Academy, the Royal Swedish Academy, the London Physical Society and the Indian Association for the Cultivation of Science in Calcutta. OSA conferred upon Wood the Frederick Ives medal for distinguished work in physical optics in 1938. Wood served as both vice-president and president of the American Physical Society.

In 1938, at the retirement age of 70, Wood changed his appointment at Johns Hopkins University from head of the physics department to research professor of physics. Two years later, the National Academy of Sciences presented him with the Draper Medal for work performed since his retirement. Wood died in Amityville, N.Y., on August 11, 1955, at the age of 87. He will always be remembered as an inventive genius who understood intuitively that creativity and science are not mutually exclusive. Indeed, to the greatest minds, they go hand in hand.

Barry R. Masters is an OSA Fellow and SPIE Fellow. He is with the department of biological engineering at the Massachusetts Institute of Technology in Cambridge, Mass., U.S.A.

References and Resources

>> R.W. Wood. “The n-Rays,” Nature 70, 530-1 (1904).
>> R.W. Wood. How to Tell the Birds from the Flowers and Other Woodcuts, Duffield and Co, 1917.
>> V. Weisskopf. “Zur Theorie der Resonanzfluoreszenz,” (Göttingen Disertation), Annalen der Physik 5(9), 23-66 (1931).
>> A.C.G. Mitchell and M.W. Zematsky. Resonance Radiation and Excited Atoms, London, Cambridge University Press, 1934.
>> W. Seabrook. Doctor Wood, Modern Wizard of the Laboratory, New York, Harcourt, Brace and Company, 1941.
>> G. Meyer-Schwickerath. Ber. Dtsch. Ophthalmol. Ges. 55, 256–9 (1949).
>> R.W. Wood. Physical Optics, Third Edition. New York, Dover Publications, 1967.
>> I.M. Klotz. “The N-Ray Affair,” Scientific American, May 1989, 168-75.
>> M.W. Jackson. ”Spectrum of Belief, Joseph Von Fraunhofer and the Craft of Precision Optics,” Cambridge, The MIT Press, 2000.
>> A. Train and R.W. Wood. The Man who Rocked the Earth, IndyPublish, 2009.

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