Dollond, John (DNB00)

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DOLLOND, JOHN (1706–1761), optician, was born at Spitalfields on 10 June 1706, of Huguenot parents, who had fled from Normandy to London on the revocation of the edict of Nantes. The conjectured original spelling of their name as d'Hollande implies that they were of Dutch extraction. Dollond was brought up to the hereditary trade of silk-weaving, and his father's death, while he was still a child, compelled the sacrifice of his education to the necessities of his family. But no impediments could debar him from self-improvement. His studies embraced Latin, Greek, anatomy, theology, no less than algebra and geometry; and his recreation at the age of fifteen consisted in solving problems, drawing figures, constructing sundials, &c. An early marriage restricted his little leisure; yet he contrived, by curtailing sleep, to attain proficiency in optics and astronomy, the subjects of his later and lasting devotion.

In 1752, his eldest son, Peter Dollond [q. v.], having set up as an optician, he abandoned silk-weaving to join him, and rapidly attained the practical skill for which his theoretical acquirements had laid the foundation. His first appearance before the learned world was in a controversy on the subject of Newton's law of refraction with Euler, who in the ‘Berlin Memoirs’ for 1747 (p. 274) had endeavoured to substitute for it a hypothetical principle permitting the colour-correction of telescopes by the employment of combined lenses of glass and water. Dollond expressed his objections in a letter to James Short [q. v.] dated 11 March 1752, which Short persuaded him to send to Euler, and communicate, with his reply, to the Royal Society. It appeared in the ‘Philosophical Transactions’ with the title ‘A Letter concerning a Mistake in M. Euler's Theorem for correcting the Aberrations in the Object-Glasses of Refracting Telescopes’ (xlviii. 289). Because Newton, on the strength of his celebrated ‘eighth experiment’ (described in his ‘Opticks,’ 3rd edit. p. 112), had despaired of correcting colour-aberration by a multiplicity of refractions, Dollond declared it to be ‘somewhat strange that anybody nowadays should attempt to do that which so long ago has been demonstrated impossible.’ A geometrical investigation by Klingenstierna, a Swedish mathematician, nevertheless showed the inconsistency with known optical phenomena of Newton's law of dispersion, the truth of which was assumed by Dollond. Upon hearing of this in 1755 he, however, decided to repeat the fundamental experiment upon which the contested principle had been made to rest. The results and the process by which they were arrived at were set forth in his memorable ‘Account of some Experiments concerning the different Refrangibility of Light,’ read before the Royal Society on 8 June 1758 (ib. l. 733). Adjusting prisms of water and glass so as to produce equal and contrary refractions, he found that the rays issued, parallel to their original direction, yet strongly coloured. The complementary experiment of producing, by similar means, refraction without colour was performed with equal success early in 1757. Object-glasses, however, constructed on this plan proved defective, owing to their short radii of curvature and consequent excessive spherical aberration, and Dollond proceeded to look out for corresponding properties in various kinds of glass. Towards the end of the same year, accordingly, he began to grind wedges of flint and crown, and apply them together so as to produce opposite refractions. His success went far beyond his anticipations. The difference in the dispersive power of the wedges thus combined was so great that an object viewed through them remained perfectly colourless when the refraction by the flint was to that by the crown in the proportion of two to three.

Thus was established the completely novel principle of the dependence of dispersion upon the quality of the refracting substance. The problem of the colour-correction of telescopes was thereby (speaking broadly) solved, but an increase of the spherical defect was a penalty which, at first sight, appeared formidable. This too, however, Dollond divined a means of removing by equalising opposite errors, ‘and thus at last,’ he concluded, ‘I obtained a perfect theory for making object-glasses, to the apertures of which I could scarcely conceive any limits’ (p. 742). Very narrow limits were, indeed, set to aperture by the backward state of the glass-making art; while the practical difficulty of working curved surfaces with the requisite precision was very great. Yet, ‘after numerous trials,’ and by ‘resolute perseverance,’ it was overcome, and refractors of the new kind, three feet in length, proved the equals of those of forty-five feet constructed by the older methods. The earliest ‘achromatics’ (a name bestowed by Dr. Bevis) had double object-glasses, but Dollond quickly perceived the advantage of dividing the biconvex crown lens into two of lower curvature, between which a biconcave flint lens was inserted. These triple objectives were, however, at first employed only with a concave eye-piece, and were rendered generally available by Peter Dollond in 1765.

The invention of the achromatic telescope was rewarded with the Copley medal in 1758, though Dollond was not then a member of the Royal Society. After his death it was found to have been anticipated. An action for infringement of patent brought by Peter Dollond in 1766 against one Champness of Cornhill was defended on the ground that Chester More Hall [q. v.] had, thirty-three years previously, made perfectly similar instruments. The fact was proved; but Lord Mansfield held that ‘as Hall had confined the discovery to his closet, and the public were not acquainted with it, Dollond was to be considered as the inventor.’ The plaintiff obtained 250l. damages, and the decision has ever since been regarded as a leading case on the subject (H. Blackstone, ii. 469; Gent. Mag. 1766, p. 102, 1790, p. 890; Ranyard, Monthly Notices, xlvi. 460).

Before working out his grand discovery, Dollond bestowed much attention on the eye-pieces of telescopes, and by a combination of five or six separate lenses succeeded in widening the field, while giving greater distinctness to the image. The particulars were embodied in a ‘Letter to Mr. James Short, F.R.S., concerning an Improvement of Refracting Telescopes,’ read before the Royal Society on 1 March 1753 (Phil. Trans. xlviii. 103). To the same body he imparted, on 10 May 1753, ‘A Description of a Contrivance for Measuring small Angles,’ and on 25 April 1754 ‘An Explanation of an Instrument for Measuring small Angles’ (ib. pp. 178, 551). This was in effect the modern heliometer. For Bouguer's twin object-glasses Dollond substituted a single one divided into two equal segments, moveable along their line of section, and the whole revolving round its optical axis. Their mutual displacement was measured by a vernier fastened to the brasswork holding one of the halves, so as to slide along a scale attached to the other. By this means he proposed to measure the spheroidal compression of the planets, the elongations of Jupiter's satellites, and the lunar diameter. Three types of ‘divided object-glass micrometer’ were indicated by him, of which only the first has held its ground. To the third, adapted to reflectors, he gave his own preference, and it was immediately carried into execution by Short, but has never proved really useful (Gill, Encycl. Brit. xvi. 250).

Towards the close of his life, Dollond occupied himself with computing almanacs for various parts of the world, one of which, for the meridian of Barbadoes, anno 1761, was possessed by his grandson, George Dollond [q. v.] Early in 1761 he was elected a member of the Royal Society, and appointed optician to the king, but his enjoyment of these honours was of brief duration. While engaged, on 30 Nov. 1761, in an intense and prolonged study of Clairaut's treatise on the motions of the moon, he was struck with apoplexy, and died in a few hours, aged 55. He left two sons and three daughters, one of whom married his celebrated apprentice, Jesse Ramsden. The only authentic account of his life was written by the husband of one of his granddaughters, Dr. John Kelly, rector of Copford, Essex, who thus described him: ‘In his appearance he was grave, and the strong lines of his face were marked with deep thought and reflection; but in his intercourse with his family and friends he was cheerful and affectionate; and his language and sentiments are distinctly remembered as always making a strong impression on the minds of those with whom he conversed. His memory was extraordinarily retentive, and amidst the variety of his reading he could recollect and quote the most important passages of every book which he had at any time perused.’

[Kelly's Life of John Dollond, privately printed, substantially reproduced in Phil. Mag. xviii 47 (1804), and in Phil. Trans. Abridg. x. 341 (Hutton), 1809; Haag's La France Protestante (2nd ed.), v. 433; Gallery of Portraits, iii. 12, with engraving by Posselwhite from a portrait of Dollond in the Royal Observatory; Gent. Mag. 1820, p. 90; Hutton's Phil. and Math. Dict.; Grant's Hist. of Phys. Astronomy, p. 531; Bailly's Hist. de l'Astr. Moderne, iii. 116; Montucla's Hist. des Math. iii. 448; Whewell's Hist. of Inductive Sciences (3rd ed.), ii. 213, 289; Brewster's Edinb. Cyclopædia, art. ‘Telescopes;’ H. Servus's Gesch. des Fernrohrs, p. 77 (Berlin, 1886); G. Fischer on Heliometer, Sirius, xvii. 176; Watt's Bibl. Brit.]

A. M. C.