Doctor Wood. Modern Wizard of the Laboratory: The Story of an American Small Boy Who Became the Most Daring and Original Experimental Physicist of Our Day-but Never Grew Up

Chapter Seven. Wood’s Work Begins

Young Professor Wood’s early and final decision to make physical optics his special field came in a curious way, toward the end of his first year at Madison. Professor Snow had asked him to undertake a graduate course of lectures on that subject — which Wood had never studied before. He willingly agreed and began to bone up, keeping just a jump ahead of his classes at first. He says that when the bell rang at the end of the hour he had just about reached the end of his knowledge of the subject. But soon he began pulling ahead. He was reading the current journals of physics and found that marvelous new fields of optics were being opened up which were not treated in the textbook he was using, Thomas Preston’s Theory of Light. By the end of the year he had done enough independent study to realize that Preston was at least ten years behind the times. So he made his decision — he would make physical optics his specialty, and he would write his own textbook!

You will have to go a long way to beat that, I think, as a piece of sheer scholastic impudence. But the joker is that he did it — and that the monumental opus stands today, in its third revised edition and translated into German, French, Russian, and other languages, as one of the world’s standard books on the subject. The book was to take five years to complete, and was not to appear until Wood had gone to Johns Hopkins, but he immediately plunged into experimental work that was to get him world-wide attention and, in the local papers, the name of the Wisconsin Wizard.

Just what was the subject that Robert Wood was choosing? Physical optics is the scientist’s name for what he practices when he combines the resources of physics and chemistry to study and learn all he can of the nature, habits, and possible uses of light. In a sense it is a science that is as old as man’s first speculation of the cause of the rainbow; but as a modern science it may be said to date from Sir Isaac Newton, who first proved that a prism simply breaks white light into its component parts, which can be reassembled again into white light. For nearly two hundred years after Newton, scientists were preoccupied with the basic characteristics of ordinary light. They measured its velocity through space. They noted how light rays were bent when they passed through other media, such as glass, quartz, water, or colored solutions, and they formulated the laws of this bending, or refraction, to give us the telescope and the microscope. They noted how light passed through a narrow slit tends to spread out, and how no shadow, when minutely examined, shows a sharp break between black and white, and they named this phenomenon diffraction. They studied also the phenomenon of interference, in which one ray of light cancels out another and complete darkness results. By the middle of the nineteenth century they knew enough about light to know that light, heat, electricity, and magnetism were allied phenomena: they were waves of energy radiating in a hypothetical medium called the ether, and they differed from one another only in their wave lengths and their frequency of vibration.

The classical theory of light had thus been rounded out long before Wood came on the scene. But vast new possibilities in physical optics had been opened up in 1859 when the spectroscope came into use for detecting the chemical nature of substances. A spectroscope is nothing more than a prism mounted between a source of light and an adjustable eyepiece (or photographic plate) for accurate observation. The prism bends each color of light that enters it at a different angle; it is the spreading out (the scientist calls it dispersion) of the component parts of white light that gives you the rainbow or solar spectrum you see in a crystal chandelier or in a spectroscope when sunlight is passed through it. But Bunsen and Kirchhoff in 1859 discovered that if, instead of passing sunlight through a spectroscope, you used the light of chemical substances heated to luminosity, you got spectra of an entirely different kind, and that the substance could be identified by its characteristic spectrum (Wood had identified the origin of the boarding-house hash by just this method).

This discovery made the spectroscope one of the major instruments of modern science, and opened almost illimitable fields for physical optics. For light became not only something to be examined in itself, but a powerful tool for examining the nature of the physical world. The minutest traces of substances revealed themselves in their spectra; and the most distant nebulae and stars showed their composition — and even their velocity and direction — in their spectra. The subject became more complicated as it developed, for it was found that the same substance gave different spectra depending on the physical state in which it was. Thus the analysis of spectra revealed not only the chemical composition of substances, but the physical condition in which they existed as well. And different types of spectra were investigated: the emission spectra of luminous bodies, and the absorption spectra emitted when light of various kinds is passed through non- luminous liquids and gases. With all this development, the task of the scientist in physical optics became that of subjecting light to every conceivable kind of test to make it tell more of its nature and the nature of its source. He studied the emission of light by luminous bodies under various types of excitation, such as sparks, electric arcs, and gases at low pressure in vacuum tubes carrying an electric current. He examined fluorescence, or the emission by certain substances of light of a different color from that of the light played upon them. He placed the source of light in powerful electric and magnetic fields. And he carried his investigations beyond the bounds of visible light to the region of the infrared and the ultraviolet and X rays.

FISH-EYE VIEWS: Photographs Wood made with his “fish-eye” camera (see here). Top: the first outdoor photograph taken – a railroad trestle seen from directly underneath. Bottom: a photograph Wood took of himself in the window of his laboratory, by putting his camera at the end of a six-foot plank and working the shutter by remote control.

MORE FISH-EYE VIEWS. Top: the camera is on the ground and a group of men stand in a circle around it. A group of fishermen standing around a small trout pool would look something like this to the trout looking upward from the bottom of the pool. Bottom: a photograph of McCoy Hall at Johns Hopkins, taken from across the street.

When Wood came on the scene at the end of the nineteenth century, physical optics was in this exciting stage of evolution. And physics in general was in one of its greatest transitional stages — the stage between the atom and Newtonian physics and the electron and Einstein. Wood’s role was to be the daring experimentalist whose work would continually challenge the formulations of the theoretical and mathematical physicists, and thus bring them closer to the ultimate truth. And equally important, his experimental demonstrations would confirm the truth of many of their purely theoretical conclusions. His first major contribution to physical optics is a beautiful example of this — and also an example of the amazing scope of his special field of science. Here is his account.

It was the total eclipse of the sun on May 28, 1900, that started me on research problems, the solution of which might be considered as contributing to knowledge in the field of physical optics. What had gone on before was for the most part along the line of demonstrations and interpretations. The Naval Observatory at Washington had invited me to become a member of its eclipse expedition and I was stationed with the group “on location” at Pinehurst, North Carolina, near the center of the belt of totality where the duration of the total phase was at its maximum. Here I had my first view of the solar corona and the red hydrogen flames blazing up at various points on the rim of the sun. The “flash” spectrum was of especial interest to me. Just before totality, when the edge of the sun is about to disappear behind the moon, one sees for a second or two a thin crescent of fire, which, if viewed through a diffraction grating or prism, is spread out into a spectrum of colored crescents, of all the colors of the spectrum, separated by dark intervals of various widths. This is the so-called chromospheric or “flash” spectrum, the chromosphere being the atmosphere of luminous metallic vapors that surround the sun. It is the absorption by this atmosphere of glowing vapor of the far brighter light of the incandescent fluid surface of the sun that produces the dark lines in the sun’s spectrum shown by the spectroscope. These lines are not absolutely black but contain the less brilliant light of the luminous vapor.

On my return to Madison in the autumn I read in the October number of the Astrophysical Journal an article by W. H. Julius, the Dutch astronomer, advancing the bold theory that the “flash” spectrum was due to anomalous dispersion of the white light originating at the fluid surface of the sun. I immediately started work to see if the “flash” spectrum could be produced in the laboratory. Before Christmas I had sent off to the Astrophysical Journal an account of a successful experimental verification of the theory of Julius. To accomplish this it would be necessary to form on a white surface an atmosphere of sodium vapor in which the density changed very rapidly as the surface was approached. This I accomplished by heating metallic sodium in an iron spoon just below the under surface of a slab of plaster of Paris, expecting that condensation of the vapor on the cold surface would produce the required change of density. The white surface on the further side of the sodium atmosphere was illuminated by an intense beam of sunlight concentrated by a large lens. This represented the white hot surface of the sun, while the sodium atmosphere represented the chromosphere. Viewing the white spot with a telescope and direct vision prism, and moving the instrument upward, thus causing the spot to become fore shortened into a line, the sun’s dark absorption lines appeared, just as they do in the case of an eclipse, when the sun’s disk is nearly covered by the moon. On moving the spectroscope until it was just inside of the plane of the illuminated surface, the solar spectrum vanished, and there suddenly blazed out two narrow yellow lines in the place occupied by the dark absorption lines of the continuous spectrum which had just vanished. Julius wrote me immediately of his delight at the outcome of this experiment, which furnished strong support for his theory. As a result of the successful outcome of this experiment I realized that a study of the optical properties of the dense absorbing vapor of metallic sodium would probably yield results of importance for the confirmation of current optical theories, and I decided to commence with a study of the dispersion of the vapor.

Here you have a beautiful example of the magnificent range of Wood’s field of physics. A man reproduces in the laboratory a model of something that is taking place ninety-two million miles away, and contributes to our knowledge of the nature of our prime source of light. The experiment is interesting in another way, for it shows an abiding characteristic of Wood’s experimental technique — his use of the simplest kind of equipment in the most daring way. You will see a lot more of this in the rest of the book: old iron pipes, abandoned bicycle parts, household bric-a-brac — all these play their parts in some of Wood’s most important work. The man has a genius for using the instrument closest to hand for his own purposes.

Wood’s work on sodium vapor and its optical properties, which began with this experiment, was to continue through most of his career. Maybe it was the small boy in Wood that made him attach himself to this substance, which has the unusual property of exploding violently when it comes in contact with water. At any rate, he set himself the task of making it yield all its secrets. In doing so he made basic contributions to our modern theories of the nature of all matter.

With sodium vapor, and also with iodine and mercury vapor, Wood was soon getting hitherto unknown types of spectra. His results gave the theoretical physicists immediate sharp pain and anguish. Without having asked their permission, this troublesome young experimenter had increased the number of spectrum lines in the principal series of sodium from the eight previously known to forty-eight, and had found a band of continuous absorption in the ultraviolet region. On the theory current at the close of the nineteenth century, each spectrum line was supposed to be emitted by a separate “vibrator” in the atom; or, as Darrow expressed it, an atom was regarded as being analogous to a clarion of bells. Rowland himself once said that the iron atom must be regarded as more complicated than a grand piano. Wood’s results made a further complication, and it was not until Niels Bohr in 1913 formulated our present theory of the nature of the atom that Wood’s results could be explained; and in Bohr’s first paper on the subject he cited Wood’s work on sodium as the most perfect confirmation of his theory of atomic radiation.

It was in Madison that Wood started another line of special interest in his field that was to stick with him for life. He became interested in the construction and uses of diffraction gratings. These are plates of glass or metal upon which have been ruled a large number of fine lines (sometimes as many as thirty thousand to the inch). Diffraction gratings perform the same function as prisms, dispersing light into its components, and for many kinds of spectroscopic work are greatly superior to prisms. Naturally their construction is a delicate task. The great Rowland made the finest gratings of his time in his laboratory at Johns Hopkins, and Wood was later to carry on and improve Rowland’s process at that institution. And as I write this he is getting ready to go to California with his chef d’œuvre!

Wood’s work with diffraction gratings had one immediate by-product that gave him wide attention while he was still in Madison — the invention of a new process of color photography which no one had previously dreamed of. It came about in a curious way. Wood had been invited by Professor Snow to a meeting of the Town and Gown Club, a select group of local potentates and faculty members which met once a month and listened patiently to an hour’s dull lecture. Membership was considered the highest honor in Madison, and it was deemed a distinction even to be invited as a guest. Apparently Wood was insensible to this honor, and smoked throughout the lecture and thought his own thoughts.

On the way home, as he and Snow were tramping through the deep snow, Wood suddenly said: “I’ve worked out all the details of a radically new process of color photography. If you take a diffraction grating, put it in front of a lens before a light, and put your eye in the green of the spectrum, the whole surface appears green. If another grating with a coarser spacing is put beside it, this grating will shine with a red light”. And all the way home through the snowstorm Wood proceeded to describe in detail the whole process, which he had thought out completely during the Town and Gown lecture.

In the spring of 1899 Wood conceived the idea of studying light waves through their analogy with sound waves — and of projecting drawings of the latter on a cinematographic screen. There were, properly speaking, no motion pictures in those days, but the primitive machine had already been invented, and Wood was the first to foresee its possibilities in connection with animated drawings[5].

He had been puzzling over what form the light wave must assume in some of the complicated processes of reflection, as, for example, in a hollow spherical mirror. It occurred to him that this question might be solved by making use of the analogy between sound and light. A German physicist named Toepler had devised an instrument by which it was possible to photograph the spherical sound wave given off by the “snap” of an electric spark. This wave is, in fact, a spherical shell of highly compressed air, which expands with velocity of more than a thousand feet per second. To catch it before it has passed out of the field of the camera, it must be illuminated by the light of a second spark which occurs at about one ten- thousandth of a second later. With Toepler’s instrument, he made a long series of photographs of sound waves undergoing reflection and refraction, as well as diffraction and dispersion. One of the photographs showed the reflection of a sound wave from a little flight of steps made of glass and placed beneath the spark. The echo from the flight of steps consisted of a train of waves and constituted a musical note of high pitch. This phenomenon, the conversion of an explosive sound into a musical note, can be verified by clapping the hands together in front of a flight of steps, if one is in the open air where no echoes from the walls or ceiling interfere with the observation of the musical note which is echoed back from the steps.

The reflection of these waves from curved surfaces was extremely complicated. He first worked out a geometrical method of constructing their forms from theory, as they went through their contortions. These evolutions he drew on paper in black ink, by the hundreds, and then photographed them one at a time in succession on motion-picture film, which had only just been put on the market. Next he obtained a machine for projection, and found that the method gave admirable results. The black line representing the sound wave moved along, twisting and folding back upon itself in curious ways, and gave a striking picture of what was actually also happening to light waves in the case of reflection of light under similar conditions. Practically all of the optical phenomena of reflection and refraction of light were reproduced by sound waves and could now be studied in a new way.

The results were communicated to scientific journals here and abroad. Also the daily newspapers, caring nothing about the analogy with light waves — which was the only thing Wood did care about — were excited by the novelty of “seeing” sound waves, and reproduced page after page of the photographed drawings.

In January, 1900, Wood received an invitation from the Royal Society of Arts asking him to come to London and deliver a lecture on his color photography at the February meeting. Then came a letter from the physicist, Sir Charles Vernon Boys, inviting him to present before the Royal Society his animated photographs of sound waves. There are many Royal Societies, Royal Astronomical, Royal Photographic, Royal Microscopical, Royal Society of Arts, and Royal What-Have-You, but there is only one Royal Society tout court — founded in 1660 and unquestionably the most important scientific body in the world. Professor Snow was greatly excited and took the matter up with President Adams, who took it up with the Regents of the University, and Wood was given a two months’ leave of absence.

Boys met him on his arrival in London, put him up at the Savile Club and the Athenaeum, and secured suitable “lodgings” for him just around the corner from the former. His lecture before the Society of Arts was on St. Valentine’s day, with Sir William Abney in the chair. But the great occasion was still to come…

The young American professor’s appearance before the Royal Society was scheduled for the following afternoon. Boys had finally located and set up a motion picture projecting machine, of which there were then only two in London.

When they entered the sacred portals, the Fellows of the Society were having tea in the noble assembly room from which they all presently proceeded to the auditorium. Lord Lister, the venerable father of antiseptic surgery, presided from a thronelike chair behind an elevated desk. The great gold mace of Cromwell’s time was brought in on a red velvet cushion and laid solemnly on the desk in front of the president. Cromwell had treated it with less formality, and his celebrated order, “Remove that bauble!” has echoed down the ages.

In the audience sat many of the great scientific celebrities then alive in London: Crookes, Dewar, Sir Oliver Lodge, and Lord Rayleigh. In a moment they would be listening to “a young man from Wisconsin”, who would be standing where stood Isaac Newton, Davy, Faraday, and all the great in Britain’s scientific history. But if you think all this overwhelmed our young man from Wisconsin, you don’t yet know him. Says he in his notes: “I showed them the sound-wave photographs and moving diagrams without a hitch and spoke extemporaneously, feeling no more embarrassment than when lecturing to my students at Madison”.

Nonsense! Actually, he was acutely aware of the tremendous honor, and he was undoubtedly boiling with excitement. For it was the dawn of world-wide fame.