DOLLOND, John, a practical and theoretical optician of the highest celebrity, the discoverer of the laws of the dispersion of light, and inventor of the achromatic telescope, descended from a family of French refugees, was born in London on the 10th June 1706.

His first destination was the manufactory which afforded employment to the greater part of the French colony established in Spittaifields, and he passed some of his earlier years in the mechanical labour of a silk-weaver. He was, however, always attached to the mathematics and to natural philosophy, and he even extended his studies to the outlines of anatomy and of scholastic divinity; and in the pursuit of these objects he found himself obliged to acquire a competent knowledge of the Latin and Greek languages, a task which was much facilitated to him by the possession of a memory no less retentive than his observation was accurate and his reasoning correct. He married early, and he continued in his first occupation till he had established his eldest son, Peter Dollond, who inherited his own tastes as an optical instrument maker; and the success of the undertaking was such as to induce him, in 1752, to leave his own business, and to enter into partnership with his son in Vine Court.

These arrangements having taken place, it was not long before Mr Dollond communicated to the Royal Society some of the results of the application of his inventive powers to his new pursuits; and Mr Short, who then enjoyed the highest reputation as an optician, paid him the compliment of bringing them forward to the Society under the auspices of his name.

1. A Letter to Mr James Short, F. R. S. concerning an Improvement of Refracting Telescopes. Phil. Trans. 1753, p. 103. The author here describes a telescope with six glasses, as calculated for correcting, either wholly or in great measure, the errors of refraction arising from the dispersion of the different colours, as well as from the spherical form of the surfaces of the eye-glasses; appealing to the superiority of the telescopes which he had thus constructed, to those which had before been in use; but he reserves a more ample detail of the theory for a future occasion, which, however, does not appear to have presented itself, the improvement having been superseded by others incomparably more important.

2. A Letter to James Short, A. Μ. F. R. S. concerning a Mistake in Mr Eulers Theorem for correcting the Aberration in the Object Glasses of Refracting Telescopes ; read 23d November 1752; together with an introductory letter of Mr Short, in which Euler’s calculations are somewhat too categorically condemned, and with Euler’s answers to Short and Dollond. Phil. Trans. 1753, p. 287. It is remarkable with what profound respect the experiments of Newton are treated in Mr Dollond’s letter: “It is somewhat strange,” he says, “that any body now-a-days should attempt to do that which so long ago has been demonstrated impossible.” But although the investigation of truth was perhaps in this instance retarded, yet its ultimate discovery was not prevented by a just deference to a high authority. Euler was, however, certainly right in considering the law which he had assumed as sufficiently compatible with the results of Newton’s experiments: although he was much mistaken in bis conjectures respect-

• ing the achromatic properties of the eye.

3. A Description of a Contrivance for Measuring Small Angles. PhU. Trans. 1753, p. 178. This apparatus consists of a divided object-glass, with a scale for determining the distance of the images, by measuring the linear displacement of the two portions of the glass, which subtends the same angle from the focus of parallel rays, as the actual distance of the images does from the object-glass. The apparatus is recommended as particularly calculated to be applied to a reflecting telescope, and was afterwards adapted by Mr Peter Dollond to the improved achromatic telescopes. Mr Savery and Mr Bouguer had before used two separate lenses in a manner nearly similar; bût the employment of a single glass divided affords a much more convenient arrangement.
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4. An Explanation of an Instrument for Measuring Small Angles. Phil. Trows. 1754, p. 551. This paper contains a more detailed theory of the divided object-glass micrometer, and a testimony of its utility from Mr Short, founded on actual experiments.

5. An Account of some Experiments concerning the Different Refrangibility of Light. Phil. Trans. 1758, p. 733. We have here the important results of a series of accurate experiments, by which the author had undertaken to investigate the foundations of the Newtonian theory of refraction; though he began them without any hope of a success so brilliant as that which ultimately crowned his labours.

It was in the beginning of 1757 that Mr Dollond made the decisive experiment of putting a common prism of glass into a prismatic vessel of water, and varying the angle of the vessel till the mean refraction of the glass was compensated; when he found that the colours were by no means destroyed, as they were supposed to have been in a similar experiment related by Newton; for the remaining dispersion was nearly as great as that of a prism of glass of half the refracting angle. Mr Dollond then employed a thinner wedge of glass, and found that the image was colourless when the refraction of the water was about one fourth greater than that of the glass. He next attempted to make compound object-glasses by inclosing water between two lenses; but in this arrangement he found great inconvenience from the spherical aberration; so that he was obliged to try the effects of different kinds of glass, and he fortunately discovered that the refractions of flint and crown glass were extremely convenient for his purpose, the image afforded by them being colourless when the angles were to each other nearly as two to three; and hence he inferred that a convex lens of crown glass and a convex one of flint would produce a colourless image when their focal distances were in the Same proportion. The spherical aberration, where the curvature was so considerable, still produced some inconvenience; but having four surfaces capable of variation, he was enabled to make the aberrations of the two lenses equal; and since they were in opposite directions, they thus corrected each other. All these arrangements required great accuracy of execution for their complete success; but, in the hands of the inventor, they produced the most admirable instruments, and he was singularly fortunate in obtaining a quantity of glass of more uniform density than has been since manufactured on so large a scale. He afterwards made some small Galilean telescopes with triple objectglasses, and Mr Peter Dollond applied this construction to the longer telescopes with compound eye-pieces, the alteration rendering the spherical aberration still more manageable.

The merits of Mr Dollond’s inventions were promptly acknowledged on the part of the Royal Society by the adjudication of the Copleyan medal for the year. In 1761, he was appointed optician to the king, and was elected a fellow of the Royal Society; a distinction which is often obtained on easy terms by those whose situation in life exempts them from the suspicion of seeking it for any purpose degrading to science, but which is generally an object of considerable ambition to persons of mechanical or commercial occupations.

A considerable share of the credit due to Mr Dollond’s discoveries has been very erroneously attributed by some late historians and biographers on the Continent to Leonard Euler, a mathematician who most assuredly has little need of the appropriation of the merits of others to establish his claim to immortality. But in fact the only idea of Euler that could be said to have furnished any hint to Mr Dollond, has been shown by the calculations of Dr Maskelyne, and by the experiments of Dr Thomas Young and Dr Wollaston, to have been completely erroneous; nor did Euler even admit the accuracy of Mr Dollond’s conclusions after his discovery was made, without considerable hesitation and scepticism. Mr Klingenstierna had simply expressed a doubt with respect to the result of Newton’s experiments, though he by no means suspected the extent of the error. Mr Peter Dollond has sufficiently vindicated his father’s claim to complete originality, in a paper read to the Royal Society in the year 1789; he has also suggested an explanation of the origin of Newton’s mistake, by stating that there exists a kind of Venetian glass, of which the dispersive power little exceeds that of water, whilst its specific gravity nearly approaches to 2∙58, which is assigned by Newton to glass in general; and it certainly seems more probable that some such circumstance as this was the cause of the error, than that Newton should, as some have suspected, have mixed acetate of lead with the water which he used, for an experiment which was so much more likely to be satisfactory without it.

Mr Dollond’s appearance was somewhat stern, and his language was impressive, but his manners were cheerful and affable. He was in the habit of attending regularly, along with his family, the service of the French Protestant church. He constantly sought his chief amusement in objects connected with the study of those sciences which he had so much contributed to improve. Perhaps, indeed, he pursued them with an application somewhat too intense; for on the 30th of November, as he was reading a new work of Clairaut on the theory of the moon, which had occupied his whole attention for several hours, he had an attack of apoplexy, which shortly became fatal. He left two sons and three daughters. His sons succeeded to his business; and the younger dying a few years afterwards, his place was filled by a nephew, who assumed the family name, and who still conducts the establishment with undiminished respectability and success. (Kelly’s Life ο; John Dollond, with an Appendix of all the Papers referred to, 3d edit. 4to, Lond. 1808.) (l. l.)