While the improvement of reflecting telescopes was in progress, the efforts to combine glass lenses in order to diminish the coloured fringes by which the images in dioptrical telescopes are surrounded were not entirely neglected ; and as early as 1729, a private gentleman, Mr. Chester More Hall, of Essex, influenced, it appears, by an opinion that the humours of the eye are combined so as to correct the dispersions which each alone would produce in the different kinds of light, con trived to combine two lenses of different kinds of glass in such a way as to form an image which was free from colours: it is added that telescopes with such object-glasses were in the possession of several individuals many years afterwards. (` Gent. Mag.', October, 1790 ; Phil. Mag.', November, 1798.) In 1747, Euler, guided also by the constitution of the eye, conceived the possibility of forming a lens compounded of two hollow spherical segments of glass, inclosing water between their concave sides, which should be free from the chromatical and spherical aberrations; and in Investigating the curvatures, he assumed that the logarithms of the terms expressing the ratio of the refraction of a mean ray in passing from air into glass, and (rein air into water, were proportional to the logarithms of the terms expressing the ratio of the refractions of red ray. in the same media. lie was nut able to obtain from any artist a 1 as of this nature, in which the 'imposed end was accomplished, and Mr. Donors!, in a short paper which Is printed in tho Philosophical Traneactiona ' (1752), contested the justness of Ruler's principle on the ground that it was contrary to one which he conceived to be founded on the experiments of Newton.
But 31. Klingen.stierna. a Swedish mathematician, having soon afterwards, in a 3Iamoire which was sent to the Acad6mie des Sciences, pointed out that the principle which had been adopted by Dollond was not conformable to the acknowledged laws of refraction, the latter determined immediately on having recourse to experiment. Either guided by the object-glasses constructed under the direction of Mr. Hall, or from a series of experiments made by himself on the refract tion of light in wedges of crown and flint glass, he discovered that by employing a convex lens of the former, in combination with a concave lens of the latter kind, the rays of the different colours in each pencil of light, after refraction through both, might be made to unite at the focus, and thus produce an image of the object nearly free from colour. For this important discovery Mr. Dollond received from the Royal Society the Copleian medal. In 1765 his son, Mr. Peter Dollond, diminished the aberration of light on account of the spherical forms of the lenses by combining together two convex lenses of crown glare with a concave lens of flint glass between them : this construction is particularly advantageous, by the increased aperture which it allows when the focal length of the compound lens is abort.
For several years after the telescopes thus improved by Dollond had been in general use, Euler continued to believe that all kinds of glass differed but little from each other with respect to their dispersive power, and he ascribed the success of the English artist merely to a fortunate determination of the curvature of his lenses; but having, in the year 1764, received information that, by the addition of lead, glass had been obtained whose dispersive power was four times as great as that of the common kind, he immediately renounced his former opinion ; and from that time the merit of the achromatic object glasses, as they were called, has been firmly established. The most eminent mathematicians, both on the Continent and in this country, have subsequently investigated, on scientific principles, the curvatures which should bo given to the surfaces of lenses, so that, the focal length of the compound lens being assumed, the chromatical and sphe rical aberrations may be corrected.
Tho arrangement of lenses for the eyepieces of telescopes is of no less importance than the formation of the objectaglaas ; and Huyghens proposed (' Dioptrics,' prop. 51), in order to diminish the refraction of light at the surfaces, to substitute for the single eyeglass of the common astronomical telescope two convex lenses, of such curvatures that the whole refraction, or the angle between the incident and emergent ray in the former construction, should be divided between the two lenses.
One mode of affecting this purpose is to place the first eye-glass, or that which is nearest to the object, so as to intercept the pencils coining from the object-glass before the rays are united, and thus the image is formed after the refraction of the light in this lens : the second eye-glass is then placed so that the rays falling on it, after having crossed at the place of the image, are made to enter the eye parallel to one another. A micrometer cannot be applied to such an eye-piece, since any change in the place of the lens which is nearest to the eye would derange its adjustment : these eye-pieces can however be rendered achromatic, and they have the greatest possible field of view ; they have therefore been constructed for the purpose of merely viewing the celestial bodice by Dollond, Ramsden, and Frauenhofer. Mr. Itarnsden was the first who constructed oye-pieces with two lenses which were capable of being used with a micrometer : this lie accom plished by placing the tube containing those lenses so that the rays in the pencils, after crowing at the focus of the objectaglass, fell in a diverging state upon the first eye-glass, and, after refraction in both, entered the eye in parallel directions.