1902 Encyclopedia > John Dollond

John Dollond
English optician

JOHN DOLLOND (1706-1761), the celebrated optician, was the son of a French refugee, a silk-weaver at Spitalfields, where he was born, June 10, 1706. He was early trained to his father's occupation, but made leisure for the acquisition of a knowledge of mathematics, physics, Greek, Latin, the elements of anatomy, and other subjects. In 1752 he abandoned silk-weaving in order to join his son Peter, who had entered upon business as an optical instrument-maker in Vine Court, and before long he became universally celebrated as an optician. His last and most important contribution to the Philosophical Transactions, for which he, in 1758, received the Copley medal of the Boyal Society, gave a description of the various experiments, begun early in 1757, on the combined effect of water and prisms and lenses of glass, by which he was led to the discovery of a means of constructing achromatic lenses. Sir Isaac Newton had stated in his Optics " that all refracting substances diverged the prismatic colours in a constant proportion to their mean refraction," and consequently " that refraction could not be produced without colour," for which reason " no improvement could be expected in the refracting telescope." Dollond, however, found that as flint glass causes a greater dispersion in proportion to its refractive power than crown glass, achromatic magnified images could be obtained by using a combination of a doubly concave lens of the former substance with a doubly convex lens of the latter. As the two glasses to be combined were the segments of spheres of considerable curvature, the aberrations from their surfaces were very great, but by varying the surfaces he was enabled to make the aberrations equal, so that, as the refractions of the two glasses were contrary, they corrected each other. In 1761 Dollond was appointed optician to the king, and became a fellow of the Boyal Society On September 30th of that year, whilst reading a work by Clairaut on the theory of the moon, he had an attack of apoplexy, of which he died in a few hours.

Dollond's published papers are "A letter to Mr James Short, F.R.S., concerning an Improvement of Reflecting Telescopes ;" a second letter to Mr Short "Concerning a Mistake in M. Euler's Theorem for Correcting the Aberration in the Object-Glasses of Refracting Telescopes ; " and "A Description of a Contrivance for measuring Small Angles "—Phil. Trans. 1753, pp. 103, 287, 178 ; " An Explanation of an Instrument for measuring Small Angles," ibid. 1754, p. 551 ; and " An Account of some Experiments concerning the different Refrangibility of Light," ibid., 1758, p. 733.—See Kelly, Life of John Dollond, 3d ed. 1808.

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