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During the Optical Revolution

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During the Optical Revolution The Debate on the "Polarity of Light" during the Optical Revolution XIANG CHEN Communicated by JED Z. BUCHWALD Introduction In the 1830s physical optics underwent a major transformation as the principles and traditions of emission (or particle) theory were displaced by those of the new undulatory (or wave) theory. Heated debates accompanied the change, during which the undulatory theory's supporters argued for its explana- tory superiority, whereas the theory's opponents presented evidence for its incompetence) Many of these debates concerned the respective physical models of the two traditions -- particles of light vs. waves in the ether -- and their abilities to account for known phenomena. Because the quantitative power of the undulatory theory seems, in retrospect, extraordinarily superior to that of its alternative, little attention has been paid either to what the alternative theory could do, or to what the undulatory theory had difficulties with. Recent work has however shown that the rival to the undulatory theory was hardly powerless, for its assumption that light consists of countable objects (rays) that gather in sets (beams) produced a great deal of quantitative work. 2 Indeed, this understanding was so powerful that many undulationists who were not for some time fully able to embody their understanding of light in a coherent apparatus frequently turned back to the apparatus of ray theory when they encountered novel phenomena, a This paper concentrates on a debate that concerned primarily the ability of undulatory principles to account for a new optical phenomenon. The debate was initiated in 1837 by DAVID BREWSTER, a stubborn opponent of the 1 For examples of the arguments for the undulatory theory, see LLOYD, 1834; for examples of the arguments against the undulatory theory, see BREWSTER, 1832. 2 For an example of a possible quantitative account of elliptic polarization from the emission tradition, see BUCHWALD, 1992, 67-74. 3 For example, even JOHN HERSCHEL had trouble in understanding the differences between ray and wave analysis. See BUCHWALD, 1989, 291-96. 360 X. CHEN undulatory theory, on the basis of a novel experimental result. Undulationists failed to explain the anomaly exposed by BREWSTER for more than a decade, and they accordingly suffered a temporary setback. The predicament of undula- tionists came not only from their failures in explanation, but from the fact that they inconsistently revised the theory's analytical apparatus, importing into it incompatible elements of ray optics. Thus, the controversy's closure was not easy to achieve: in the process undulatory techniques and tools themselves underwent further development. We will see that many of the salient issues in this debate remained below the surface, hidden beneath the explicit points regarding physical models or ex- planatory abilities. Indeed, it is precisely for this reason that the debate lasted as long as it did. We will see, in particular, that BREWSTER succeeded for so long because he based his reasoning tacitly on ray conceptions, which enabled him to characterize his discovery as a new property of light, in effect of a new optical kind. Undulationists did not recognize BREWSTER'S tacit apparatus, and they attempted to deal with his discovery by applying new tools that they developed specifically to deal with this special situation. Yet undulationists themselves reasoned initially in terms of rays, (albeit not as BREWSTER did) and not in terms of wave-fronts. It took years of experience, as well as people who trained from the beginning in the undulatory tradition (and who therefore had not been influenced by the rival form of optics), before the anomaly was removed. We will also see that the early undulatory analyses in this debate were significantly influenced by the specific experimental design, which appeared to be strikingly similar to interference experiments, and these could often be dealt with by ray analysis. The anomaly however resisted such a treatment, and it was in fact successfully dealt with only when interference methods, where rays may work, were replaced by the methods of diffraction, which require integra- tion over wave fronts. The successful classification and explanation of the anomaly were finally achieved on the basis of a new experimental device that produced the same result but in a different way, quickly and decisively remov- ing the irritant. This episode vividly shows that a full historical understanding of scientific debates requires examinations of not only articulated arguments but also of the many tacit factors that influence scientific practice, such as classifica- tion, the analytical apparatus deployed, and the specifics of experimental design. 1. The discovery Fox TALBOt, an undulationist, discovered an interesting phenomenon in prismatic spectra in 1837. In this experiment, TALBOT used a prism with moder- ate dispersive power to produce a spectrum. Instead of observing the spectrum directly, he inserted a circular aperture the size of the pupil just in front of the eye, and he covered one half of the aperture with an extremely thin plate of glass (Fig. 1). When he viewed the prismatic spectrum in this way, TALBOT observed a group of parallel dark bands crossing throughout the spectrum, similar to those produced by absorption. He attributed the bands to interference. "Polarity of Light" during the Optical Revolution 361 Thin plate ~] The eye Aperture ]~ Prism Fig. 1. TALBOT'Sexperimental design. Relying on HUYGENS' principle, TALBOT reasoned that every point in the spec- trum acted as a new source, emitting homogeneous rays that were focused by the crystalline lens of the eye onto the retina. Since the rays passing through the upper half of the lens experienced retardation caused by the plate, they could interfere with the unretarded rays that passed through the other half of the lens. When the retardation, which varied according to the color of a ray, was an odd number of half wavelengths, the light would be extinguished by interference. The spectrum would thereby be interrupted by dark bands. 4 TALBOT did not, however, attempt to produce a quantitative account. TALBOT'S discovery was published in the Philosophical Magazine in 1837, and it drew the attention of DAVID BREWSTER, who was a vociferous opponent of the undulatory theory. BREWSTER had begun to study optics as early as 1799, and he had an established reputation in experimental optics by the mid 1810s. Although his early optical researches were deeply influenced by the emission tradition, BREwsTER never publicly admitted that he was an emissionist, nor was he willing to give a straightforward answer to the question of whether light is particles or waves, s BREWSTER'S optical researches, however, always contained an implicit element that was thoroughly consonant with the emission tradition: he always thought of light as rays, and he analyzed optical phenomena in terms of their properties, occasionally thinking in terms of the deflection of rays by forces emanating from material bodies. The late 1830s was a difficult period for opponents of the undulatory theory in Britain. Wave optics dominated the journals after mid-decade, and the theory had strong advocates among the most prominent "gentlemen of science" of the 4 TALBOT, 1837, 364. TALBOT's explanation was problematic, because it also predicted the formation of bright bands (due to constructive interference) that had not been observed. TALBOT probably based his explanation on ARAGO's account of stellar scintillation, which attributed the momentary disappearance of starlight to interference between rays that passed the two halves of the eye's lens or the telescope's objective. s See BREWSTER, 1835, 1-2. 362 X. CHEN day, including GEORGE AIRY, the Astronomer Royal, BADENPOWELL, the Savilian Professor of geometry at Oxford, and HUMPHREY LLOYD, professor of natural and experimental science at Trinity College, Dublin. Its opponents in Britain did not however immediately surrender to the new system, and several of them continued to fight a rearguard action. BREWSTER, for one, worked hard to collect experimental results that were inconsistent with, or at least unexplained by, the undulatory theory. Foremost among these anomalies were dispersion and absorption, because undulationists themselves had difficulties here. 6 Thus, when TALBOT reported that his prismatic experiment produced an absorption- like effect, BREWSTER immediately thought to use the experiment in a further attack. BREWSTER repeated TALBOT'S experiment, but with two significant revisions. First, instead of using the naked eye, he examined the spectrum formed at the focus of an achromatic telescope, thereby producing a distinct and sharp spec- tral image. The existence of dark bands in prismatic spectra was well-known in the 1830s. Using a telescope to view the spectrum, FRAUNHOFERin the 1820s has reported the existence of more than 600 dark lines in the light from the sun. Since these dark lines could not be seen by the naked eye, the telescope was taken to be a standard device in line-observing experiments. BREWSTER thus added an achromatic telescope to TALBOT'S design. Because the object tens of the telescope in itself functioned as an aperture limiting the amount of light allowed to pass, BREWSTER did not use TALBOT'S circular aperture. He kept the thin plate of glass directly in front of his eye, covering one half of the pupil (Fig. 2). BREWSTER thereby made TALBOT'S dark bands more intense and distinct. BREWSTER'S second change to TALBOT'S design was to rotate the thin plate during the experiment. In his early experiments on polarization by refraction, conducted in the mid 1810s, BREWSTER found that observational results varied when the analyzer -- usually a piece of agate -- was rotated: some images disappeared altogether when the agate was turned to a particular angle with the plane of refraction. 7 Given the experience gleaned from this much-earlier work on polarization, BREWSTER decided to try rotating the thin plate in the modified TALBOT experiment.
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