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*, 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 of D'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

*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.

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.

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 objectglasses 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 page 209; 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 the divided object-glass micrometer, one of the most useful instruments for measuring small angles.

But the fame of Dollond principally rests upon his invention of achromatic, 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 the dispersion of light in passing through a refracting medium, that is, the greater or less length through which the coloured spectrum is scattered, is not in proportion to the refraction, 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*. 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 accord

*See Penny Cyclopædia, article Achromatic, for this and other terms employed in this Life.

ingly turned his attention entirely to the reflecting telescope.

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 be nearly true; 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