The principal development of the achromatic refractor is due to John Dollond who invented it independently (Phil Trans., 1758). In principle his object-glasses were of the pattern mainly used at the present day; viz., convex lens of crown glass combined with a concave lens of flint glass. The concave lens is of less power than the convex, so that the combination converges the light as a single convex lens would do ; but the flint glass having much wider difference of refractive index for light of different colours, is able to correct the colour dispersion introduced by the more powerful crown lens. A triple objective, consisting of two convex lenses of crown glass with a concave lens of flint glass between them, was introduced in 1765 by Peter, son of John Dollond. This type is also employed in some modern telescopes. The subsequent improvement of both reflecting and refracting telescopes has been mainly a matter of technical detail in the making, figuring, and mounting of large lenses and mirrors. At present the largest refractor has an aperture of 4o in. (Yerkes
observatory), and the largest reflector an aperture of ioo in. (Mt. Wilson observatory). A limit to the aperture of refractors is set by the difficulty of casting large enough discs of glass of the neces sary transparency and homogeneity ; moreover, if this were over come, we should ultimately reach a point at which the strain on the lens caused by its own weight would spoil its optical qualities. The size of reflectors appears to be limited only by consideration of expense, not only of making the mirror, but of providing a mounting for its practical use. Funds have been provided for the construction of a reflector of 200 inches aperture, and it is ex pected that the instrument will be built in the course of a few years. Pictures of typical modern telescopes are given in the plate illustrating the article OBSERVATORY. Introductory remarks on the general use of astronomical telescopes will be found under ASTRONOMY : Practical Astronomy.