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BLAIR, ROBERT, M.D. (d. 1828), inventor of the ‘aplanatic’ telescope, was born (there is reason to believe) at Murchiston, near Edinburgh. He was, in all probability, identic with the Robert Blair who wrote ‘A Description of nn accurate and simple Method of adjusting Hadley‘s Quadrant for the Back Observation,’ appended to the ‘Nautical Almanac’ for 1788 (published 1783), and printed separately by order of the commissioners of longitude. But the first fact authentically known about him is his appointment by a royul commission, dated 25 Sept. 1785, to the chair of practical astronomy erected for his benefit in the university of Edinburgh, with a yearly salary of 120l.. Being unprovided with instruments or an observatory, he held the post as a complete sinecure for forty-three years, eight of which he is said to have spent in London, where his only son, Archibald Blair, was established as an optician. When in Edinburgh he rarely entered the Seuatusrkcademiciis, and his name was even omitted from the list of professors furnished to the university commission, which began its sittings in 1826. In 1787 Blair undertook, with a view to Ending a substitute for flint glass, the first systematic investigation yet attempted of the dispersive powers of various media, the results of which were lengthly detailed in a paper read before the Royal Society of Edinburgh 3 Jan. and 4 April 1791. He was the first to attempt the removal of the ‘secondary spectrum,' and succeeded in his attempt by a triple combination of two essential oils, such as naphtha and oil of turpentine, with crown glass; but his discovery of fluid media possessing the same relative, though a different absolute dispersion from glass, gave a far more brilliant prospect of practical success. This valuable optical property he found to belong to metallic solutions, especially of antimony and mercury, mixed with chlorhydric acid, and to the absolutely colourless refraction thus rendered possible he gave the name of ‘aplanatic’ or ‘free from aberration’ (Ed. Phil. Trans.. iii. 53). ‘Could solid media of such properties be discovered,’ Sir John Herschel remarked (Encycl. Metr. iv. 429), ‘the telescope would become a new instrument.’ Blair constructed object-glasses upon this principle, of which the performance was highly praised, in one case, at least, venturing successfully upon the unexampled feat of giving to an aperture of three inches a focal length of only nine. He took out a patent for his invention, and entrusted the fabrication of the new instruments to a London optician, George Adams the younger [q. v.]; but they never came into general use. An equally fruitless effort to establish a regular manufacture and sale of them in Edinburgh was made by Archibald Blair, under his father's directions, in 1827 (Ed. Journ. of Science, vii. 336). The fluid used in the lenses appears, in course of time, to have lost its transparency by evaporation or crystallisation, and the difficulty offered by the secondary spectrum is, by modern art, rather evaded than overcome.

Sir David Brewster relates (Enycl. Brit. art. ‘Optics,’ p. 586, eighth edition) that an instrument for magnifving by means of prisms, similar to the ‘teinoscope’ invented by himself in 1812 (Ed. Phil. Journ. vi. 334), was shown him by Archibald Blair as having been constructed by his father at an unknown date. The principle of the contrivance was arrived at independently by Amici of Modena in 1821.

Blair became a fellow of the Royal Society of Edinburgh in January 1796, and at one period held the appointment of first commissioner of the board for the care of sick and wounded seamen. In this capacity he was instrumental in banishing scurvy born the navy by introducing the use of lime-juice, a method of preserving which for an indefinite time at sea he had previously ascertained (Ed. Journ. of Science, vii. 341). In 1827 he published at Edinburgh a small volume, entitled ‘Scientific Aphorisms, being the outline of an attempt to establish fixed principles of science, and to explain from them the general nature of the constitution and mechanism of the material system, and the dependence of that system upon mind.’ The large promise of the title-page is but imperfectly fulfilled by the contents. Extending Lesage's machinery for producing the effects of gravitation, he divided matter into three classes, distinguished by the size of the constituting ‘protected,’ ‘jaculatory,’ and ‘quiescent’ particles, in the mutual collisions of which he sought a universal explanation of phenomena of the material order, all motion being, however, in the last resort, referred to the action of mind. His health was by this time much broken, and he died at Westlock, in Berwickshire, 22 Dec. 1828.

An abridgment of his ‘Experiments and Observations on the unequal Refrangibility of Light,’ originally published in the ‘Transactions of the Royal Society of Edinburgh’ (iii. 3-76, 1794), appeared in Nicho1son’s ‘Journal of Natural Philosophy’ with the title, ‘The Principles and Application of a new Method of constructing Achromatic Telescopes’ (i. 1, 1797), and, in a German translation, in Gilbert’s ‘Annalen der Physik’ (vi. 129, 1800). The best account of the principle of his ‘fluid lens,’ or aplanatic telescopes, will be found in Sir John Hersche1's article on Light in the ‘Encyclopaedia Metropolitana’ (pars. 474-7).

Sir Alexander Grant's Story of the University of Edinburgh (1884), i. 339, ii. 361; Cat. of Scientific Papers, i. 1867.]

A. M. C.