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.