Dollond, John

, an eminent optician, and the inventor of the achromatic telescope, was born in Spitalfields, June 10, 1706. His parents were French protestants, and at the time of the revocation of the edict of Nantz, in 1685, resided in Normandy, but in what particular part cannot now be ascertained. M. de Lalande does not believe the name to be of French origin; but, however this may be, the family were compelled soon after this period to seek refuge in England, in order to avoid persecution, and to preserve their religion. The fate of this family was not a solitary case; fifty thousand persons pursued the same measures, and we may date from this period the rise of several arts and manufactures, which have become highly beneficial to this country. An establishment was given to these refugees, by the wise policy of our government, in Spitalfields, and particular encouragement granted to the silk manufactory.

The first years of Mr. Doliond’s life were employed at the loom; but, being of a very studious and philosophic turn of mind, his leisure hours were engaged in mathematical pursuits; and though by the death of his father, which happened in his infancy, his education gave way to the necessities of his family, yet at the age of fifteen, before he had an opportunity of seeing works of science or elementary treatises, he amused himself by constructing sun-dials, drawing geometrical schemes, and solving problems. An early marriage and an increasing family afforded him little opportunity of pursuing his favourite studies but such are the powers of the human mind when called into action, that difficulties, which appear to the casual observer insurmountable, yield and retire before perseverance and genius; even under the pressure of a close application to business for the support of his family, he found time, by abridging the hours of his rest, to extend his mathematical knowledge, and made a considerable proficiency in optics and astronomy, to which he now principally devoted his attention, having, in the earlier | stages of his life, prepared himself for the higher parts qjf those subjects by a perfect knowledge of algebra and geometry.

Soon after this, without abating from the ardour of his other literary pursuits, or relaxing from the labours of his profession, he began to study anatomy, and likewise to read divinity; and finding the knowledge of Latin and Greek indispensably necessary towards attaining those ends, he applied himself diligently, and was soon able to translate the Greek Testament into Latin; and as he admired the power and wisdom of the Creator in the mechanism of the human frame, so he adored his goodness displayed in his revealed word. It might from hence be concluded that his sabbath was devoted to retired reading and philosophical objects; but he was not content with private devotion, as he was always an advocate for social worship, and with his family regularly attended the public service of the French protestant church, and occasionally heard Benson and Lardner, whom he respected as men, and admired as preachers. 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 recollected as always making a strong impression on the minds of those with whom he conversed. His memory was extrordinarily 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.

He designed his eldest son, Peter Dollond, for the same business with himself; and for several years they carried on their manufactures together in Spital -fields; but the employment neither suited the expectations nor disposition of the son, who, having received much information upon mathematical and philosophical subjects from the instruction of his father, and observing the great value which was set upon his father’s knowledge in the theory of optics by professional men, determined to apply that knowledge to the benefit of himself and his family; and, accordingly, under the directions of his father, commenced optician. Success, though under the most unfavourable circumstances, attended every effort; and in 1752, John Dollond, embracing the opportunity of pursuing a profession congenial with his mind, and without neglecting the rules of | prudence towards his family, joined his son, and in consequence of his theoretical knowledge, soon became a proficient in the practical parts of optics.

His first attention was directed to improve the combination of the eye-glasses of refracting telescopes; and having succeeded in his system of four eye-glasses, he proceeded one step further, and produced telescopes furnished with five eye-glasses, which considerably surpassed the former; and of which he gave a particular account in a paper presented to the royal society, and which was read on March 1, 1753, and printed in the “Philosophical Transactions,” vol. XLVIII. Soon after this he made a very useful improvement in Mr. Savery’s micrometer for, instead of employing two entire eye-glasses, as Mr. Savery and M. Bouguer had done (see Bouguer), he vised only one glass cut into two equal parts, one of them sliding or moving laterally by the other. This was considered to be a great improvement, as the micrometer could now be applied to the reflecting telescope with much advantage, and which Mr. James Short immediately did. An account of the same was given to the royal society, in two papers, which were afterwards printed in the “Philosophical Transactions,” vol. XLVIII. This kind of micrometer was afterwards applied by Mr. Peter Dollond to the achromatic telescope, as appears by a letter of his to Mr. Short, which was read in the royal society Feb. 7, 1765.

Mr. Dollond’s celebrity in optics became now universal; and the friendship and protection of the most eminent men of science, flattered and encouraged his pursuits. To enumerate the persons, both at home and abroad, who distinguished him by their correspondence, or cultivated his acquaintance, however honourable to his memory, would be only an empty praise. Yet among those who held the highest place in his esteem as men of worth and learning, may be mentioned, Mr. Thomas Simpson, master of the royal academy at Woolwich; Mr. Harris, assaymaster at the Tower, who was at that time engaged in writing and publishing his “Treatise on Optics;” the rev. Dr. Bradley, then astronomer royal; the rev. William Ludlam, of St. John’s college, Cambridge and Mr. John Canton, a most ingenious man, and celebrated not less for his knowledge in natural philosophy, than for his neat and accurate manner of making philosophical experiments. To this catalogue of the philosophical names of those days, | we may add that of the late venerable astronomer-royal, the rev. Dr. Maskelyne, whose labours have so eminently benefited the science of astronomy.

Surrounded by these enlightened men, in a state of mind prepared for the severest investigation of philosophic truths, and in circumstances favourable to liberal inquiry, Mr. Dollond engaged in the discussion of a subject, which at that time not only interested this country, but all Europe. Sir Isaac Newton had declared, in his Treatise on Optics, p. 112, “That all refracting substances diverged the prismatic colours in a constant proportion to their mean refraction,” and drew this conclusion, “that refraction could not be produced without colour,” and consequently, “that no improvement could be expected in the refracting telescope.” No one doubted the accuracy with which sir Isaac Newton had made the experiment; yet some men, particularly M. Euler and others, were of opinion that the conclusion which Newton had drawn from it went too far, and maintained that in very small angles refraction might be obtained without colour. Mr. Dollond was not of that opinion, but defended Newton’s doctrine with much learning and ingenuity, as may be seen by a reference to the letters which passed between Euler and Dollond upon that occasion, and which were published in the “Philosophical Transactions,” vol. XLVIII.; and contended, that, “if the result of the experiment had been as described by sir Isaac Newton, there could not be refraction without colour,”

A mind constituted like Mr. Dollond‘ s, could not remain satisfied with arguing in this manner, from an experiment made by another, but determined to try it himself, and accordingly in 1757 began the examination; and, to use his own words, with “a resolute perseverance,” continued during that year, and a great part of the next, to bestow his whole mind on the subject, until in June 1758 he found, after a complete course of experiments, the result to be very different from that which he expected, and from that which sir Isaac Newton had related. He discovered “the difference in the dispersion of the colours of light, when the mean rays are equally refracted by different mediums.” The discovery was complete, and he immediately drew from it this practical conclusion, “that the objectglasses of refracting telescopes were capable of being made without the images formed by them being affected by the | different refrangibility of the rays of light.” His account of this experiment, and of others connected with it, was given to the royal society, and printed in their Transactions, vol. L. and he was presented in the same year, by that learned body, with sir Godfrey Copley’s medal, as a reward of his merit, and a memorial of the discovery, though not at that time a member of the society. This discovery no way affected the points in dispute between Euler and Dollond, respecting the doctrine advanced by sir Isaac Newton. A new principle was in a manner found out, which had no part in their former reasonings, and it was reserved for the accuracy of Dollond to have the honour of making a discovery which had eluded the observation of the immortal Newton. The cause of this difference of the results of the 8th experiment of the second part of the first book of Newton’s Optics, as related by himself, and as it was found when tried by Dollond in 1757 and 1758, is fully and ingeniously accounted for by Mr. Peter Dollond in a paper read at the royal society, March 21, 1789, and afterwards published in a pamphlet.

This new principle being now established, he was soon able to construct object-glasses, in which the different refrangibility of the rays of light was corrected, and the name of achromatic was given to them by the late Dr. Bevis, on account of their being free from the prismatic colours, and not by Lalande, as some have said. As usually happens on such occasions, no sooner was the achromatic telescope made public, than the rivalship of foreigners, and the jealousy of philosophers at home, led them to doubt of its reality and Euler himself, in his paper read before the academy of sciences at Berlin in 1764, says, “I am not ashamed frankly to avow that the first accounts which were published of it appeared so suspicious, and even so contrary to the best established principles, that I could not prevail upon myself to give credit to them;” and he adds, *’ I should never have submitted to the proofs which Mr. Dollond produced to support this strange phenomenon, if M. Clairaut, who must at first have been equally surprized at it, had not most positively assured me that Dpllond’s experiments were but too well founded.“And when the fact could be no longer disputed, they endeavoured to find a prior inventor, to whom it might be ascribed; and several conjecturers were honoured with the title of discoverers. But Mr. Peter Dollond in the paper we have | just mentioned, has stated and vindicated, in the most unexceptionable and convincing manner, his father’s right to the first discovery of this improvement in refracting telescopes, as well as of the principle on which it was founded. In so doing he has corrected the mistakes of M. de la Lande in his account of this subject; those of M. N. Fuss, professor of mathematics at St. Petersburg, in his” Eulogy on Euler,“written and published in 1783; and those of count Cassini, in his” Extracts of the Observations made at the Royal Observatory at Paris in the year 1787;" and it must appear to every impartial and candid examiner, that Mr. Dollond was the sole discoverer of the principle which led to the improvement of refracting telescopes.

This improvement was of the greatest advantage in astronomy, as they have been applied to fixed instruments; by which the motions of the heavenly bodies are determined to a much greater exactness than by the means of the old telescope. Navigation has also been much benefited by applying achromatic telescopes to the Hadley’s Sextant; and from the improved state of the lunar tables, and of that instrument, the longitude at sea may now be determined by good observers, to a great degree of accuracy; and their universal adoption by the navy and army, as well as by the public in general, is the best proof of the great utility of the discovery.

In the beginning of 1761, Mr. Dollond was elected F. R. S. and appointed optician to his majesty, but did not live to enjoy these honours long; for on Nov. 30, in the same year, as he was reading a new publication of M. Clairaut, on the theory of the moon, and on which he had been intently engaged for several hours, he was seized with apoplexy, which rendered him immediately speechless, and occasioned his death in a few hours afterwards. His family, at his death, consisted of three daughters and two sons, Peter and John, who, possessing their father’s abilities, carried on the optical business in partnership, until the death of John, when it was continued, and still flourishes, under the management of Mr. Peter Dollond, well known as an able philosopher and artist, and Mr. George Huggins, his nephew, who, upon the king’s permission, has taken the name of Dollond.¹