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
