BLAIR, ROBERT

(b. Murchiston [near Edinburgh], Scotland, 1748; d. Westlock, Berwickshire, Scotland, 22 December 1828)

optics, maker of optical instruments.

Blair was an expert in optics and a maker of optical instruments. His main contributions were twofold. First, throughout his adult life he improved the performance of achromatic prisms and lenses. Second, he wrote an important paper on the classical relativistic optics of moving bodies. The manuscript was never published, but it provides an essential demonstration of the application of Newton’s dynamics to light and a most interesting foreword to Einstein’s relativity. In this context, after John Michell (on whose work he based his efforts), he discovered what sixty years later would be named the Doppler effect.

Career. Robert Blair was the son of Archibald Blair, a minister in Garvald, Scotland, and his wife, Janet Barclay. Robert was born at Murchiston, near Edinburgh. After studying medicine at Edinburgh University, he was apprenticed as a naval surgeon and served in the West Indies, where he became interested in navigation and its instruments.

In 1785 Blair was appointed the first professor of practical astronomy at the University of Edinburgh. He was awarded the degree of master of divinity. His chair at Edinburgh, a regius one, was created for him by the Edinburgh Town Council for “the great advantages which Navigation … derive from the cultivation of Practical Astronomy” (Brück, 1983, p. 11). In 1786 he became a fellow of the Royal Society of Edinburgh. At the university, Blair “enjoyed forty two years of endowed leisure”: he refused to give lectures on the ground that he had neither apparatus nor observatory and did not attend meetings of the university senate (Grant, 1884, 2, p. 362). He resided for eight years in London working with his son Archibald, also an optician. After 1793, Blair held the appointment of first commissioner of the board for the care of sick seamen. He was instrumental in banishing scurvy from the navy by introducing the use of lime juice. After a long illness he died at Westlock, Berwickshire, on 22 December 1828.

Blair published few articles: one on Hadley’s quadrant, one on optics, and another on the construction of achromatic telescopes. In 1786 he wrote a paper that would remain unpublished. (The manuscript was read on 6 April 1786 at the Royal Society. It was communicated by Alexander Aubert (1730-1805), fellow of the Royal Society and director of the London Assurance company.) In it he gave a systematic treatment of the Newtonian kinematics of light, taking into account in the absolute space of Newton the motion of the light source, that of the observer, and the velocity of the corpuscles of light.

Later he published two books, Essays on Scientific Subjects(1818) and Scientific Aphorisms(1827). He was well known for his work on achromatism and one of those who contributed through research to the Newtonian optics of moving bodies.

Doppler Effect. Blair’s most important contribution to the optics of moving bodies follows from a deep understanding of how Newton’s dynamics were applied to light at the end of the eighteenth century. At the time it was not clear that the velocity of light was a constant; in fact, because of Galileo’s kinematics it could not be constant. Newton’s Principia (1687) proposed a dynamics of particles whose application to gravitation was highly successful. But its application to light—the corpuscular theory of light—is also of interest.

It consists of a short range of dynamics of light that essentially implied the sinus law of refraction. Blair was to follow John Michell’s analysis of the corpuscular theory of light and took seriously what Michell called then his method, using a prism as a tool to measure the velocity of light. The following result stems from a simple argument in the refraction model of the corpuscular theory: the greater the velocity of an incident light corpuscule on a crystal, the smaller is its angle of refraction. Thus, a measure of the refraction angle is a measure of the incident velocity of the light corpuscule. But from Galileo’s kinematics, which then had of necessity to apply to a light corpuscule, the incident velocity of a light corpuscule should be the sum of the emission’s velocity (supposed to be a constant) and the relative velocity between the source and the observer. Thus, from a measure of the refraction suffered by a light ray emitted by a moving body, one gets a measure of the velocity of the body relative to the observer. From such an analysis, Blair reasonably thought that he would be able to determine “the motion with which any planet, Comet or fixed star however remote, is approaching toward, or receding from the observer” (Blair, 1786, p. 10). This is precisely the essence of what has come to be called the Doppler effect. Blair constructed an instrument in order to make such measures, but for different reasons (he used twelve achromatic prisms and absorption was far too important due to the number of prisms), he was unable to make any measurement. Blair’s proposal is at the root of François Arago’s well-known experiments from 1806 into the 1810s on the velocity of light, whose influence on the acceptance of Augustin-Jean Fresnel’s undulatory theory of light has been so important. Most probably Arago never read Blair’s manuscript but he heard of it through an article written by John Robison. (John Robison, “On the Motion of Light, as affected by Refractiong and Reflecting Substances, which are also in motion,”Royal Society of Edinburgh. Transactions 2 (1790): 83–111, p. 98, which is quoted in Arago’s article).

Moreover, in the context of the undulatory theory of light, Blair proposed an experiment to determine the absolute motion of the Earth, laying the basis for the famous experiment performed by Albert Michelson one hundred years later. Actually Blair’s unpublished manuscript contains the very basic questions of light relativity and the roots of spectroscopy. It addresses precisely the problems that would be hotly debated in the nineteenth century, only to be solved by Albert Einstein in 1905.

Achromatism . In his Opticks(1704), Newton had erroneously asserted that chromatic aberration could not be corrected by combining two lenses of differing refracting indices. The quest for achromatism is well known; in the eighteenth century chromatic aberration was much reduced by John Dollond and many others.

At the turn of the nineteenth century, the insufficient quality of flint glass was one of the most serious problems facing opticians. While at Edinburgh, Blair sought to improve the performance of refracting telescopes and hoped to substitute a dense fluid for flint glass. The most satisfactory system was a solution of antimony of mercury in hydrochloric acid sealed between two crown glass lenses, a system he called aplanetic, meaning free from aberration.

In 1791 Blair reported his work at two meetings of the Royal Society of Edinburgh and published a paper, “Experiments and Observations on the Unequal Refrangibility of Light,” in its Transactions in 1794. In it he detailed alternative methods of constructing achromatic telescopes. He combined two oils with very different dispersive powers. The secundary spectrum (a term Blair coined) was greatly reduced but still appeared. Blair’s work was the origin of the technology of liquid-lens telescopes, which David Brewer, Archibald Blair, and Peter Barlow developed in the 1820s in response to Joseph von Fraunhofer’s superior achromatic telescopes. For Great Britain, it was one of the attempts made to regain world hegemony in optical lenses.

BIBLIOGRAPHY

Blair’s unpublished manuscript, “A Proposal for Ascertaining by Experiments whether the Velocity of Light Be Affected by the Motion of the Body from Which it is Emitted or Reflected,”Royal Society Manuscript, L & P, 8, 182, 1786, is housed in the Royal Society in London.

WORKS BY BLAIR

“A Description of an Accurate and Simple Method of Adjusting Hadley’s Quadrant for the Back Observation.” In Nautical Almanac. London: William Richardson, 1788).

“Experiments and Observations on the Unequal Refrangibility of Light.”Royal Society of Edinburgh, Transactions 3 (1794): 3–76.

“The Principles and Application of a New Method of Constructing Achromatic Telescopes.” Journal of Natural Philosophy 1 (1797): 1–13.

“Beschreibung einer neuen Art von achromatischen Fernröhren, oder der sogenannten aplanetischen Teleskope, und Entwickelung der Gründe, vorauf sie beruhen.” Annalen der Physik 6 (1800): 129–148.

Essays on Scientific Subjects. Edinburgh, UK: Macredie, 1818. Scientific Aphorisms, Being an Outline of an Attempt to EstablishFixed Principle of Science. Edinburgh, UK, n.p., 1827.

OTHER SOURCES

Brewster, David. “Observations on the Superiority of Achromatic Telescopes with Fluid Object-Glasses, as Constructed by Dr. Blair.” Edinburgh Journal of Science 5 (1826): 105–111.

Brück, Hermann A. The Story of Astronomy in Edinburgh from itsBeginnings until 1975. Edinburgh, U.K. Edinburgh University Press, 1983.

Cantor, Geoffrey N. Optics after Newton: Theories of Light inBritain and Ireland, 1704–1840. Manchester, U.K. Manchester University Press, 1983.

Eisenstaedt, Jean. Avant Einstein : Relativité, lumière, gravitation. Paris: Seuil, 2005.

———. “Light and Relativity, a Previously Unknown Eighteenth-Century Manuscript by Robert Blair (1748–1828).”Annals of Science 62 (2005): 347–376.

Grant, Alexander. The Story of the University of Edinburgh during Its First Three Hundred Years. 2 vols. London: Longmans, Green, 1884.

Jackson, Myles W. Spectrum of Belief: Joseph von Fraunhofer and the Craft of Precision Optics. Cambridge, MA: MIT Press, 2000.

Jean Eisenstaedt