Brewster, David

(b. Jedburgh, Scotland, 11 December 1781; d. Allerly, Melrose, Scotland, 10 February 1868)

optics.

The son of Margaret Key and James Brewster, rector of the Jedburgh grammar school, David entered the University of Edinburgh in 1794. Although he completed the prescribed courses, like other students of the time, he did not take a bachelor’s degree. Continuing at the university as a divinity student, he was awarded an honorary M. A. in 1800, and in 1804 he was licensed to preach in the Church of Scotland, although he was never ordained a minister. As an evangelical, Brewster became an adviser to the leaders of the Disruption; was long a friend of its leader, Thomas Chalmers; and became a member of the Free Church of Scotland in 1843.

His formal training, however, was but one part of his education. As amanuensis to Dr. Thomas Somerville, scholar, author, and minister of Jedburgh, Brewster acquired in his youth the writing and editing skills that later were his principal source of income. More important, as a child Brewster began to learn about physical science from his father’s manuscript notes from the University of Aberdeen. Encouraged and assisted by the “peasant astronomer” James Veitch, Brewster built sundials, microscopes, and telescopes. While studying at the university, he continued to build instruments and to exchange astronomical observations with Veitch. Stimulated by his classmate Henry Brougham, Brewster began his experimental researches on light about 1798. While he believed that he had disproved part of Newton’s explanation of the “inflexion” of light, Brewster did not then, or ever, abandon the emission theory of light.

Brewster’s income depended on his literary, rather than his scientific, efforts. He was a private tutor from 1799 to 1807; he edited the Edinburgh Magazine and Scots Magazine from 1802 to 1806, the Edinburgh Encyclopaedia from 1807 to 1830, and various scientific journals from 1819 to the end of his life; and he was author of numerous popular books and articles. Throughout his life he lost no opportunity to deplore the lack of paid careers for British scientists, and his repeated failure to obtain a professorship only increased his calls for reform. Moreover, he aggressively promoted scientific education for all groups in society, in the hope that wider knowledge of science would lead to its increased prestige. A leader in the establishment of the Edinburgh School of Arts (1821), the Royal Scottish Society of Arts (1821), and the British Association for the Advancement of Science (1831), Brewster attempted to create organizations, as well as opinions, that would diffuse and promote science and education. Yet with all his effort, he failed in his great plan to establish a national institute patterned on the ideas of Bacon and Newton and on the practice of France.

Brewster was a reform Whig. His political friends, including Brougham, Charles Grant (Lord Glenelg), and the earl of Buchan (who recommended Brewster for the LL.D. at Aberdeen), were sources of influence and honor. Not only did Brougham, as lord chancellor, assist in founding the British Association, but he also obtained pensions and knighthoods for Brewster (1832) and other scientists. Through political influence, two of Brewster’s sons became East Indian officials, and in 1838 Brewster was relieved of financial worries by the government’s gift of the principalship of the United Colleges of St. Salvator and St. Leonard, St. Andrews. In 1859 Brewster was elected principal of the University of Edinburgh by the town council. Shortly thereafter he also became vice-chancellor.

Brewster received numerous honors, including the LL.D. from Aberdeen (1807) and the D.C.L. from Oxford (1832), fellowships of the Royal Societies of Edinburgh and of London, and a foreign associateship in the French Institute (1848). His awards included the Copley, Rumford, and Royal medals of the Royal Society of London and the Keith Prize of the Royal Society of Edinburgh.

Brewster’s interest in astronomy and instruments is evident in his first major scientific publication, A Treatise on New Philosophical Instruments (1813). In the latter sections of the book, he reported his determinations of the refractive and dispersive powers of nearly two hundred substances that he had made in a quest for improvement of achromatic telescopes. Not until much of this work was completed did he learn, in 1811 or 1812, that in 1808 Malus had discovered that reflected light acquired the same polarization as one of the doubly refracted beams in Iceland spar. This information, combined with his discoveries that doubly refracting bodies have two dispersive powers, that the single beam transmitted by agate is polarized, and that noncrystallized bodies such as mica “depolarise” light, shifted Brewster’s concern from instruments back to optical theory. In exploring the consequences of these experiments, Brewster followed four separate but related lines of research.

Since Brewster had found that light was partially polarized by oblique refraction in mica, he attempted to determine the law of this polarization in the simpler case of successive refractions by a pile of thin glass plates. By the end of 1813 he had concluded that “the number of plates in any parcel, multiplied by the tangent of the angle, at which it [completely] polarises light, is a constant quantity.” More important, since “the pencil of light polarised by transmission [comports] itself, in every respect like one of the pencils formed by double refraction,” study of the physical optics of transparent bodies ought to enable philosophers “to unfold the secrets of double refraction, to explain the forms and structure of crystallised bodies, and to develope the nature and properties of that etherial matter, which… performs… a capital part in the operations of the material world.” Nothing less was at stake than understanding of the structure of organized matter and the nature of light. In conceiving of his results in this way, Brewster defined much of his optical career for the next twenty years.

Brewster’s second line of study was a search for the law of polarization by reflection. While Malus had concluded that the “polarising angle neither follows the order of the refractive powers nor that of the dispersive forces,” Brewster was not convinced. His own “measures for water and the precious stones afforded a surprising coincidence between the indices of refraction and the polarising angles; but the results for glass formed an exception, and resisted every method of classification.” Persisting, however, he concluded that chemical changes on the surface of the glass had obscured the general law that “the index of refraction is the tangent of the angle of polarisation.”

Moreover, analogously to polarization by successive refractions, successive reflections at any angle continuously increased the quantity of polarized light in the beam. Now making explicit his adoption of an emission theory of light, Brewster explained his results in terms of “polarising forces” that acted on light particles “in every state of positive and negative polarisation from particles completely polarised to particles not polarised at all.” Each successive reflection brought the particles nearer to complete polarization. By late 1829 he was convinced not only that his particulate theory was the simplest possible, but also that it was fully adequate to account quantitatively for the intensities and resultant angles of polarization of reflected and refracted light. He regarded “all the various phenomena of polarisation of light by reflexion and refraction as brought under the dominion of laws as well determined as those which regulate the motions of the planets.” In 1816 he received the Copley Medal, in 1819 two Rumford Medals, and in 1831 a Royal Medal for the papers in which he announced these discoveries. On the popular level, Brewster’s reputation was established in 1816 by the fad for his kaleidoscope. Its invention was a direct result of his studies of the ory of polarization by multiple reflections.

Study of metallic reflection was a third line of Brewster’s research. Using successive reflections to increase the degree of polarization, he concluded that light reflected by metals was neither plane nor circularly—but elliptically—polarized. Moreover, from his results he deduced laws that not only accurately predicted the quantities and angles of polarization of light, but also were the foundation for theoretical researches on metallic reflection by MacCullagh and Cauchy.

Pursuing his fourth line of research, Brewster created the new fields of optical mineralogy and photoelasticity. In 1813, while studying the “depolarising” action of topaz, he observed two sets of elliptical rings (interference patterns) centered on axes in the topaz that were apparently inclined at 65°. He interpreted this to mean that topaz must have two axes, not one, of double refraction, an entirely unexpected result. After many laborious experiments he was able in 1819 to group all but a few of hundreds of minerals and crystals into mutually consistent optical and mineralogical categories: the primitive form determined the number of axes of double refraction.

During these investigations, Brewster quite unexpectedly observed that heat and pressure could produce or change a doubly refracting structure in uncrystallized, crystallized, or organic bodies. Moreover, from the geometry of the interference patterns he deduced equations that permitted him to predict the shapes, numbers, and colors of patterns that would be produced by changes in configuration, temperature, pressure, and method of observation.

In an attempt to improve colored eyeglasses and microscopy, in 1821 Brewster began an intensive study of absorption spectroscopy. Ironically, these researches led Newton’s biographer to a profound dissent from Newton’s doctrine of colors and to a strong reaffirmation of a “Newtonian” emission theory of light.

In examining a blue eyeglass spectroscopically, he concluded that it caused extreme eye fatigue by transmitting only red and blue rays. Their differing refrangibilities prevented the eye from accommodating to one focal distance. Less than a year later, in utilizing his new monochromatic microscope illuminator in studies of other absorption spectra, Brewster entered the debate over the number of colors in the spectrum: was it seven, as Newton had held, or four, as Wollaston believed, or some other number? Since Wollaston had asserted that yellow was merely a combination of green and red, Brewster first examined the solar spectrum with red-and green-absorbing glasses. Rather than the reds and greens vanishing, “the space from which the colours were absorbed was in both cases occupied by yellow light….” While this established, for Brewster, the separate existence of yellow, it invalidated Newton’s identification of color and refrangibility, for “Yellow light… has its most refrangible rays mixed with green light of equal refrangibility, and its least refrangible rays mixed with red light of equal refrangibility.” Further experiments in which he examined a salted candle flame with these filters led Brewster to the startling conclusion that, while yellow light had an independent existence, “the prism is incapable of decomposing that part of the spectrum which [yellow] occupies.” By 1831 he had extended this interpretation to the entire spectrum. However formed, it “consists of three spectra of equal length, beginning and terminating at the same points, viz. a red spectrum, a yellow spectrum, and a blue spectrum.” Moreover, a certain amount of undecomposable white light exists at every point in the spectrum.

This line of research led to Brewster’s most effective defense of an emission theory of light. In an attempt to establish techniques for optical chemical analysis, he turned to a detailed examination of the action on the spectrum of plant juices, gases, and the earth’s atmosphere. Not only did he succeed in identifying bodies by their characteristic dark lines, but he also added some 1,600 dark lines to Fraunhofer’s 354. Impressed by the extremely selective absorption of light by “nitrous acid gas,” he concluded that he could “form no conception of a simple elastic medium so modified by the particles of the body which contains it, as to make such an extraordinary selection of the undulations which it stops or transmits.”

After the 1830’s Brewster directed his attention to such subjects as photography, stereoscopy, and the physiology of vision. At the same time he began to emphasize his writing rather than his editing. His biographies of Newton, Galileo, Tycho Brahe, and Kepler; his numerous articles for the Encyclopaedia Britannica; and his hundred or more major essay reviews were, for the most part, written after 1830. His time for research was limited, and he had largely achieved his original research goals. Also, optics was increasingly dominated by an unwelcome theory, the undulatory theory of light.

Brewster never wholeheartedly accepted the undulatory theory. His most honored papers had either been based on the emission theory or had been directed to its defense. Using that theory, he had derived mathematical laws that successfully explained and predicted phenomena. At the same time he frankly admitted his admiration for the “singular power of [the undulatory theory] to explain some of the most perplexing phaenomena of optics….” However, “the power of a theory… to explain and predict facts is by no means a test of its truth….” But more important for Brewster, the undulatory theory, based on a hypothetical ether that he could not conceive of in physical terms, and that in principle never could be observed, was fatally “defective as a physical representation of the phaenomena of light….”

While Brewster repeatedly asserted the value of theory and was a competent, if not brilliant, mathematician, he was above all an experimenter. And the experimenter, not the oretician, could achieve “true” knowledge. Most important, however, Brewster believed that the undulatory theory, a mere speculation, had been raised to the level of an assumed Truth. As a devout evangelical Presbyterian who believed in the unity of truth, he felt that such unbridled speculation in physics had profoundly serious implications for religion. To him, “Speculation engenders doubt, and doubt is frequently the parent either of apathy or impiety.” To accept the undulatory theory of light would have required Brewster to abandon his deepest convictions about man’s ability to know the world and man’s duty to serve God.

BIBLIOGRAPHY

I. Original Works. There is no collected edition of Brewster’s works, nor is there a bibliography. An appendix in the Home Life (see below) reproduces the bibliography of his articles given in the Royal Society’s Catalogue of Scientific Papers. It is very incomplete and has some inaccuracies. Many other articles by him of varying importance are printed in the various periodicals that he edited, especially the Edinburgh Philosophical Journal and the Edinburgh Journal of Science. Most of his anonymous, and very revealing, essay reviews are listed in Walter E. Houghton, ed., The Wellesley Index to Victorian Periodicals, 1824–1900, Vol. I (Toronto, 1966). Brewster’s own collection of papers was accidentally burned early in the twentieth century, but important manuscript collections of his letters exist at the British Museum (Charles Babbage and MacVey Napier papers), University College, London (Henry, Lord Brougham papers), the National Library of Scotland, and the Royal Society, London.

Information in this article is based upon the following of Brewster’s own books and articles: A Treatise on New Philosophical Instruments, for Various Purposes in the Arts and Sciences. With Experiments on Light and Colours (Edinburgh-London, 1813); “On some properties of Light,” in Philosophical Transactions of the Royal Society, 103 (1813), 101–109; “On the affections of Light transmitted through crystallized bodies,” ibid., 104 (1814), 187–218; “On the Polarisation of Light by oblique transmission through all Bodies, whether crystallized or uncrystallized,” ibid., 219–230; “Results of some recent experiments on the properties impressed upon Light by the action of Glass raised to different temperatures, and cooled under different circumstances,” ibid., 436–439; “Experiments on the depolarisation of light as exhibited by various mineral, animal, and vegetable bodies, with a reference of the phenomena to the general principles of polarisation,” ibid., 105 (1815), 29–53; “On the effects of simple pressure in producing that species of crystallization which forms two oppositely polarised images, and exhibits the complementary colours by polarised light,” ibid., 60–64; “On the laws which regulate the polarisation of Light by reflexion from transparent bodies,” ibid., 125–159; “On new properties of heat as exhibited in its propagation along glass plates,” ibid., 106 (1816), 46–114; “On the communication of the structure of doubly-refracting crystals to glass, muriate of soda, fluor spar, and other substances by mechanical compression and dilation,” ibid., 156–178; “On the Effects of Compression and Dilation in altering the Polarising Structure of Doubly Refracting Crystals,” in Transactions of the Royal Society of Edinburgh, 8 (1818), 281–286; “On the Laws which regulate the Distribution of the Polarising Force in Plates, Tubes, and Cylinders of Glass that have received the Polarising Structure,” ibid., 353–372; “On the laws of Polarisation and double refraction in regularly crystallised bodies,” in Philosophical Transactions of the Royal Society, 108 (1818), 199–273; “On the connection between the primitive forms of Crystals, and the number of their Axes of double refraction,” in Memoirs of the Edinburgh Wernerian Society, 3 (1817–1820), 50–74; A Treatise on the Kaleidoscope (Edinburgh, 1819); “Observations on Vision through Coloured Glasses, and on their application to Telescopes and Microscopes of great magnitude,” in Edinburgh Philosophical Journal, 6 (1822), 102–107; “Description of a Monochromatic Lamp for Microscopical purposes, &c. with Remarks of the Absorption of the Prismatic Rays by coloured Media,” in Transactions of the Royal Society of Edinburgh, 9 (1823), 433–444; “On the production of regular double refraction in the molecules of bodies by simple pressure; with observations on the origin of the doubly refracting structure,” in Philosophical Transactions of the Royal Society, 120 (1830), 87–96; “On the law of the partial polarisation of light by reflexion,” ibid., 69–84; “On the laws of the polarisation of light by refraction,” ibid., 133–144; “On the Phenomena and Laws of Elliptic Polarisation, as exhibited in the Action of Metals upon Light,” ibid., 287–326; “Decline of Science in England and Patent laws,” in Quarterly Review, 43 (Oct. 1830), 305–342; “Observations on the Decline of Science in England,” in Edinburgh Journal of Science, 5 (July 1831), 1–16; “On a New Analysis of Solar Light, indicating three Primary Colours, forming Coincident Spectra of equal length,” in Transactions of the Royal Society of Edinburgh, 12 (1834), 123–136; “Report on the recent Progress of Optics,” in Report of the British Association for the Advancement of Science (1831–1832), pp. 308–322; “Observations of the Absorption of Specific Rays, in reference to the Undulatory Theory of Light,” in London, Edinburgh, and Dublin Philosophical Magazine and Journal of Science, 2 (1833), 360–363; “Life and Correspondence of Sir James Edward Smith,” in Edinburgh Review, 57 (April 1833), 39–69 (the quotation on “Speculation” is on p. 41); “Observations on the Lines of the Solar Spectrum, and on those produced by the Earth’s Atmosphere, and by the action of Nitrous Acid Gas,” in Transactions of the Royal Society of Edinburgh, 12 (1834), 519–530; “On the Colours of Natural Bodies,” ibid., 538–545; “Arago Éloge historique de Baron Fourier,” in North British Review, 4 (Feb. 1846), 380–412 (contains his plan for a national institute, particularly pp. 410–412); and Memoirs of the Life, Writings, and Discoveries of Sir Isaac Newton, 2 vols. (Edinburgh, 1855). The Memoirs… of Sir Isaac Newton has been reprinted in 2 volumes (New York, 1965) with a useful introduction by Richard S. Westfall that discusses both Brewster’s life and his interpretation of Newton.

II. Secondary Literature. Very little contemporary secondary material on Brewster exists, nor is he adequately treated in the standard histories of optics. The only biography that exists is Margaret Maria Gordon (Brewster), The Home Life of Sir David Brewster, 2nd ed., rev. (Edinburgh, 1870). While it is generally reliable, it is frankly directed to the “unscientific” reader. It is the source for most of his obituaries. Brewster himself particularly recommended, as very full and complete, the article “Brewster, David,” in the Biographie universelle et portative des contemporaines, V (Paris, 1836), 77–81. It has biographical information not included in the Home Life.

The standard, although very one-sided, histories of the emission-undulation controversy are Ernst Mach, The Principles of Physical Optics, an Historical and Philosophical Treatment (New York, 1953); Vasco Ronchi, Histoire de la lumière, translated by Juliette Taton (Paris, 1956); and Sir Edmund Whittaker, A History of the ories of Aether and Electricity, Vol. I. The Classical Theories (New York, 1960). Brewster’s work in optical mineralogy is briefly treated in John G. Burke, Origins of the Science of Crystals (Berkeley, 1965). A very useful discussion of Brewster’s optical theories is contained in Henry Steffens, “The Development of Newtonian Optics in England, 1738–1831” (unpublished Master’s dissertation, Cornell University, 1965).

Edgar W. Morse

Brewster, Sir David (1781-1868)

Famous nineteenth-century scientist whose brief investigation of Spiritualism in 1855 led to bitter public acrimony. Brewster was born on December 11, 1781, in Jedburgh, Scotland. His formal education was as a divinity student at the University of Edinburgh, but while there, he maintained and interest in physical science which he had become interested in as a child. The controversy was sparked when Brewster attended a séance with a Lord Brougham in Cox's Hotel in Jermyn Street in London's West End. The medium was D. D. Home, to whom Brewster was introduced by Lord Brougham. Reportedly both were deeply impressed. Home subsequently wrote to a friend in America, describing the visit and stating that they were unable to account for the phenomena by natural means. The letter, published and commented upon in America, found its way into the London press. Sir David Brewster who, in the meantime, had had another séance at Ealing in the house of Mr. Rymer, a London solicitor, promptly wrote to the Morning Advertiser, forcefully disclaiming all belief in Spiritualism and ascribing all the phenomena to imposture. His letter ended: "I saw enough to satisfy myself that they could all be produced by human hands and feet."

A heated newspaper controversy arose. Edward W. Cox, sergeant-at-law, who was present at the séance, wrote to the Morning Advertiser, to contradict Brewster, and citing Brewster's expression of astonishment: "This upsets the philosophy of fifty years." When Brewster replied that he had not been allowed to look under the table, both Cox and the well-known author T. A. Trollope (brother of novelist Anthony Trollope) also present at the Ealing séance, contradicted him. Yet another statement, one by Benjamin Coleman, quoting Sir David Brew-ster's admission of the reality of the phenomena in private conversation, was published.

Brewster replied in an angry tone, gave a description of the sitting, and declared: "Rather than believe that spirits made the noise, I will conjecture that the raps were produced by Mr. Home's toes, and rather than believe that spirits raised the table, I will conjecture that it was done by the agency of Mr. Home's feet, which were always below it." He further said that the spirits were powerless above the table but were very active beneath a large round table with copious drapery, beneath which nobody was allowed to look. After describing how a handbell from the neighborhood of Mr. Home's feet came across and placed itself into his and afterward into Lord Brougham's hands, he concluded: "How these things were produced neither Lord Brougham nor I could say, but I conjecture that they may be produced by machinery attached to the lower extremities of Mr. Home."

Throughout this passionate controversy Lord Brougham preserved an inflexible silence. Brewster never appealed to him. D. D. Home, on the other hand, challenged Lord Brougham's testimony. This was half promised but not given. However, a conversation is recorded by Cox in his book The Mechanism of Man (1876), in which he claimed that Lord Brougham stated to him: "We were both perfectly satisfied at the time that it was no trick, and that some unknown power was in action." I said 'Well, Brewster, what do you think of it?' and he said only 'There are more things in heaven and earth, Horatio, than are dreamt of in our philosophy.' "Lord Brougham also declared that Brewster never told him that he had changed his opinion. The only reason why he himself did not pursue the investigation was that he was then deeply immersed in experiments in optical science and did not have the necessary leisure.

The late earl of Dunraven, in his preface to the original private edition of Lord Adare's records on his experiences with D. D. Home, expressed the belief that Brewster acted out of fear of ridicule. He wrote: "He was present at two séances of Mr. Home's where he stated as is affirmed on the written testimony of persons present, his impression that the phenomena were most striking and startling, and he does not appear then to have expressed any doubt of their genuineness, but he afterwards did so in an offensive manner. I mention this circumstance because I was so struck with what Sir David Brewster— with whom I was well acquainted—had himself told me, that it materially influenced me in determining to examine thorough-ly into the reality of the phenomena."

In Home's Incidents in My Life (1863), Home wrote that Brewster treated certain of his scientific contemporaries even worse than he treated Home, claiming the credit for other people's inventions. Brewster threatened a libel action but despite Home enlarging the evidence in the second edition of his book, Brewster never carried out his threat.

The final word in this public debate was uttered in 1869 when The Home Life of Sir David Brewster was published, after his death in February 1868, by his daughter, Mrs. Gordon. A note is printed from the private diary of the scientist, which narrated the phenomena he witnessed in company with Lord Brougham:

"Last of all I went with Lord Brougham to a séance of the new spirit-rapper, Mr. Home, a lad of twenty, the son of a brother of the late Earl Home. He lives in Cox's Hotel, Jermyn Street; and Mr. Cox, who knows Lord Brougham, wished him to have a séance and his Lordship invited me to accompany him in order to assist in finding out the trick. We four sat down at a moderately-sized table, the structure of which we were invited to examine. In a short time the table shuddered, and a tremulous motion ran up all our arms; at our bidding these motions ceased and returned. The most unaccountable rappings were produced in various parts of the table; and the table actually rose from the ground when no hand was upon it. A larger table was produced and exhibited similar movements. A small hand-bell was then laid down with its mouth on the carpet: and, after lying for some time, it actually rang when nothing could have touched it. The bell was then placed on the other side, still upon the carpet, and it came over to me and placed itself in my hand. It did the same to Lord Brougham. These were the principal experiments. We could give no explanation of them and could not conjecture how they could be produced by any kind of mechanism."

The version from Brewster's posthumous book conflicts with his letter to the Morning Advertiser, in which Sir David expressly stated that the bell did not ring and that the table "appeared" to rise. A detailed comparison of the two statements reveals many other discrepancies. The Spectator stated in its review of Home's book, "The hero of science does not acquit himself as we could wish or expect."

There is no doubt that Brewster came out of the affair badly. He was guilty of misrepresentation when he refused to stand by his original puzzlement at the séance, and thereby was criticized for later contradicting himself. What he actually exclaimed at the time was typical of the last ditch materialist unable to believe his own senses: "Spirit is the last thing I will give in to!"