DOLLOND.

The parents of this eminent discoverer in optics, to whom we are chiefly indebted for the high perfection of our telescopes, were French Protestants resident in Normandy, whence they were driven by the revocation of the edict of Nantes in 1685. With many others of their class, they took up their residence in Spitalfields, where John Dollond, the subject of this memoir[1], was born, June 10, 1706. It has been supposed, and among others by Lalande, that the name is not French; if we were to hazard a conjecture, we should say that it might have been an English corruption ofD’Hollande. While yet very young, John Dollond lost his father; and he was obliged to gain his livelihood by the loom, though his natural disposition led him to devote all his leisure hours to mathematics and natural philosophy. Notwithstanding the cares incumbent upon the father of a family (for he married early) he contrived to find time, not only for the above-mentioned pursuits, but for anatomy, classical literature, and divinity. He continued his quiet course of life until his son, Peter Dollond, was of age to join him in his trade of silk-weaving, and they carried on that business together for several years. The son, however, who was also of a scientific turn, and who had profited by his father’s instructions, quitted the silk trade to commence business as an optician. He was tolerably successful, and after some years his father joined him, in 1752.

1.For the details of this life, we are mostly indebted to the Memoir of Dr. Kelly, his son-in-law, from which all the existing accounts of Dollond are taken. This book has become very scarce, and we are indebted for the opportunity of perusing it to the kindness of G. Dollond, Esq.

Engraved by J. Posselwhite.DOLLOND.From an original Picturein the Royal Observatory, Greenwich.Under the Superintendance of the Society for the Diffusion of Useful KnowledgeLondon, Published by Charles Knight, Ludgate Street, & Pall Mall East.

The first improvement made by the elder Dollond in the telescope,was the addition of another glass to the eye-piece, making the whole number of glasses in the instrument (the object-glass included) six instead of five. This he communicated to the Royal Society in 1753, through his friend James Short, well known as an optician and astronomer, who also communicated all his succeeding papers. By his new construction, an increase in the field of view was procured, without any corresponding augmentation of the unavoidable defects of the instrument. In May, 1753, Dollond communicated to the Royal Society his improvement of the micrometer. In 1747 Bouguer proposed to measure the distance of two very near objects (the opposite edges of a planet, for example) by viewing them through a conical telescope, the larger end of which had two object-glasses placed side by side, the eye-glass being common to both. The distance of the objects was determined by observing how far it was necessary to separate the centres of the object-glasses, in order that the centre of each might show an image of one of the objects. Mr. Dollond’s improvement consisted in making use of the same object-glass, divided into two semicircular halves sliding on one another, as represented in the diagrams in page18; the first of which is an oblique perspective view of the divided glass, and the second a side view of the same, in such a position, that the images of the stars A and B coincide at C.

If the whole of an object-glass were darkened, except one small portion, that portion would form images similarly situated to those formed by the whole glass, but less illuminated. Each half of the object-glass, when separated from the other, forms an image of every object in the field; and the two images of the same object coincide in one of double brightness, when the halves are brought together so as to restore the original form. By placing the divided diameter in the line of two near objects, A and B, whose distance is to be measured, and sliding the glasses until the image of one formed by one half comes exactly into contact with the image of the other formed by the other half, the angular distance of the two objects may be calculated, from observation of the distance between the centres of the two halves. This last distance is measured on a scale attached to the instrument; and when found, is the base of the triangle, the vertex of which is at C, and the equal sides of which are the focal lengths of the glasses. This micrometer Dollond preferred to apply to the reflecting telescope; his son afterwards adapted it to the refracting telescope; and it is now, under the name of thedivided object-glass micrometer, one of the most useful instruments for measuring small angles.

But the fame of Dollond principally rests upon his invention ofachromatic, or colourless telescopes, in which the surrounding fringe of colours was destroyed, which had rendered indistinct the images formed in all refracting telescopes previously constructed. He was led to this practical result by the discovery of a principle in optics, that thedispersionof light in passing through a refracting medium, that is, the greater or less length through which the colouredspectrumis scattered, is not in proportion to therefraction, or angle through which the rays are bent out of their course. Newton asserted that he had found by experiments, made with water and glass, that if a ray of light be subjected to several refractions, some of which correct the rest, so that it emerges parallel to its first direction, the dispersion into colours will also be corrected, so that the light will be restored to whiteness. This is not generally true: it is true if one substance only be employed, or several which have the same, or nearly the same,dispersive power[2]. Mr. Peter Dollond afterwards satisfactorily explained the reason of Newton’s mistake, by performing the same experiment with Venetian glass, which, in the time of the latter, was commonly used in England; from which he found that the fact stated by Newton was true, as far as regarded that sort of glass. Had Newton used flint glass, he would have discovered that dispersion and refraction are not necessarily corrected together: he would then have been led to the difference between refractive and dispersive power, and would have concluded from his first experiment that Venetian glass and water have their dispersive powers very nearly equal. As it was, he inferred that the refracting telescope could never be entirely divested of colour, without entirely destroying the refraction, that is, rendering the instrument no telescope at all; and, the experiment being granted, the conclusion was inevitable. It is well known that he accordingly turned his attention entirely to the reflecting telescope.

2.See Penny Cyclopædia, article Achromatic, for this and other terms employed in this life.

In 1747, Euler, struck by the fact that the human eye is an achromatic combination of lenses, or nearly so, imagined that it might be possible to destroy colour by employing compound object-glasses, such as two lenses with an intermediate space filled with water. In a memoir addressed to the Academy of Berlin, he explained his method of constructing such achromatic glasses, and proposed a new law of refrangibility, different from that of Newton. He could not, however, succeed in procuring a successful result in practice.Dollond, impressed with the idea that Newton’s experiment was conclusive, objected to Euler’s process in a letter to Mr. Short; which the latter persuaded the author to communicate, first to Euler, and then, with his answer, to the Royal Society. Assuming Newton’s law, Dollond shows that Euler’s method would destroy all refraction as well as dispersion. The latter replies, that it is sufficient for his purpose that Newton’s law should benearlytrue; that the theory propounded by himself does not differ much from it; and that the structure of the eye convinces him of the possibility of an achromatic combination. Neither party contested the general truth of Newton’s conclusion.

A new party to the discussion appeared in the field in the person of M. Klingenstierna, a Swedish astronomer, who advanced some mathematical reasoning against the law of Newton, and some suspicions as to the correctness of his experiment. The latter being thus formally attacked, Mr. Dollond determined to repeat it, with a view of settling the question, and his result was communicated to the Royal Society in 1758. By placing a prism of flint glass inside one of water, confined by glass planes, so that the refractions from the two prisms should be in contrary directions, he found that when their angles were so adjusted, that the refraction of one should entirely destroy that of the other, the colour was far from being destroyed; “for the object, though not at all refracted, was yet as much infested with prismatic colours, as if it had been seen through a glass wedge only, whose refracting angle was near thirty degrees.” It was thus proved that the correction of refraction, and the correction of dispersion, are not necessarily consequent the one on the other. Previously to communicating this result, Dollond had, in 1757, applied it to the construction of achromatic glasses, consisting of spherical lenses with water between them: but finding that the images, though free from colour, were not very distinct, he tried combinations of different kinds of glass; and succeeded at last in forming the achromatic object-glass now used, consisting of a convex lens of crown, and a concave of flint glass. His son afterwards, in 1765, constructed the triple object-glass, having a double concave lens of flint glass in the middle of two double convex lenses of crown glass. The right of Dollond to the invention has been attacked by various foreign writers, but the point seems to have been decided in his favour by the general consent of later times. His conduct certainly appears more philosophical than that of either of his opponents. So long as he believed that Newton’s experiment was correct, he held fast by it, not allowing any mathematicalreasoning to shake his belief, and in this respect he was more consistent than Euler, who seems to have thought that an achromatic combination might be made out of the joint belief of an experiment, and of an hypothesis utterly at variance with it. And the manner in which the distinguished philosopher just mentioned received the news of Dollond’s invention, appears singular, considering the side which each had taken in the previous discussion. Euler, who had asserted the possibility of an achromatic lens, against Dollond, who appeared to doubt it, 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.” Dollond was the first who actually resorted to experiment, and he thus became the discoverer of a remarkable law of optics; while his tact in the application of his principles, and the selection of his materials, is worthy of admiration. The reputation of Dollond rests upon the discovery of the law, and its application to the case in point; for it has since been proved that he was not absolutely the first who had constructed an achromatic lens. On the occasion of an action brought for the invasion of the patent, the defendant proved that about the year 1750, Dr. Hall, an Essex gentleman, was in the possession of a secret for constructing achromatic telescopes of twenty inches focal length: and a writer in the Gentleman’s Magazine for 1790, has advanced his claim with considerable circumstantial detail. It is difficult to get any account of that trial, as it is not reported in any of the books. At least we presume so, from not finding any reference to it either in the works of Godson or Davis on Patents, though the case is frequently mentioned; or in H. Blackstone’s report of Boulton and Watt v. Bull, in which Dollond’s case forms a prominent feature of the argument. But, from the words of Judge Buller in the case just cited, it is difficult to suppose that the account given by Lalande (Montucla,Histoire des Mathématiques, vol. iii. p. 448, note) can be correct. Lalande asserts that it was proved that Dollond received the invention from a workman who bad been employed by Dr. Hall, and that the latter had shown it to many persons. Judge Buller says, “The objection to Dollond’s patent was, that he was not the inventor of the new method of making object-glasses, but that Dr. Hall had made the same discovery before him. But it was holden that as Dr. Hall hadconfined it to his closet, and the public were not acquainted with it, Dollond was to be considered as the inventor.” The circumstances connected with the discovery, particularly the previous investigationof the phenomenon on which the result depends, independently of the words of Judge Buller, quoted in italics, appear to us to render the anonymous account very improbable: nor, as far as we know, is there any other authority for it. That Dr. Hall did construct achromatic telescopes is pretty certain; but we are entirely in the dark as to whether he did it on principle, or whether he could even construct more than one sort of lens: and the assertion that he, or any one instructed by him, had communicated with Dollond, is unsupported by any thing worthy the name of evidence. We may add, that the accounts of this discovery, written by Dollond himself, possess a clearness and power of illustration, which can result only from long and minute attention to the subject under consideration.

After this great discovery, for which he received the Copley medal of the Royal Society, Mr. Dollond devoted himself to the improvement of the achromatic telescope, in conjunction with his other pursuits. We are informed by G. Dollond, Esq., that his grandfather, at the latter end of his life, was engaged in calculating almanacs for various parts of the world; one of which, for the meridian of Barbadoes, and the year 1761, is now in his possession.

Mr. Dollond was elected a fellow of the Royal Society in 1761. In the same year, November 30, he was struck with apoplexy, while attentively engaged in reading Clairaut’s Theory of the Moon, which had then just appeared. He died in a few hours afterwards, in the fifty-sixth year of his age. His son Peter Dollond, already mentioned, continued the business in partnership with a younger brother; and it is now most ably carried on by his daughter’s son, who has, by permission, assumed the name of Dollond.

The following extract is from the memoir written by Dr. Kelly, in which we find nothing to regret, except that so few traits of character are related in it. Those who write memoirs of remarkable men from personal knowledge, should remember that details of their habits and conversation will be much more valuable to posterity, than disquisitions upon their scientific labours and discussions, which, coming from the pens of friends or relations, will always be looked upon asex partestatements. Had the learned author borne this in mind, we should have been able to give a better personal account of Dollond than the following; which is absolutely the only information relative to his private character which we can now obtain. “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, whomhe 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 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.”

Engraved by W. Holl.JOHN HUNTER..From a Picture by Sir Joshua Reynoldsin the Royal College of Surgeons, London.Under the Superintendance of the Society for the Diffusion of Useful Knowledge.London, Published by Charles Knight, Pall Mall East.

JOHN HUNTER.

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