Fig. 53 represents the body of an ordinary compound microscope with its triplet object-glasses; o is an object, above it is the triple achromatic object-glass, in connection with the eye-piece _e e, f f_ the plano-convex lenses; _e e_ being the eye-glass, and _f f_ the field-glass, between which, at _b b_, the arrow represents the diaphragm. The course of the light is shown by three rays drawn from the centre, and three from each end of the object _o_; these rays, if not prevented by the lens _f f_, and the diaphragm _b b_, would form an image at _a a_; but here, as they meet with the lens _f f_ in their passage, are converged by it at _b b_, the diaphragm at _b b_ intercepting a portion of peripheral rays, permitting only those to pass that are necessary for the formation of the image, the further magnification of which is, however, here brought about by the eye-glass _e e_, precisely as if it were that of the original object under examination. It will be apparent, then, that the field-lens _f f_ belongs in principle to the object-glass, or objective, taking a share in the image-forming rays, although this is taken to be a part of the eye-piece. Evolution of the Modern Achromatic Microscope. The great advances made in the optical arrangements of the modern microscope necessitated important changes and improvements in its several mechanical parts. Indeed, as the apertures of objectives became increased, and focal planes became correspondingly shallower, it was absolutely necessary to apply a more sensitive system of focussing than that for many years past commonly in use. The leading manufacturers at once grasped the situation, and in a short space of time the older model microscopes were discarded, and replaced by instruments better in workmanship and finish, and in every way more suitable for the student and the promotion of original scientific research. From an early period English amateurs appear to have bestowed greater attention on the improvement of the microscope than those of any other country. Between 1820 and 1835 Tully, Pritchard, Dolland, James Smith, Andrew Ross, and Hugh Powell, encouraged by Wollaston, Brewster, Goring, Herschel, and Lister, worked out innumerable combinations of single and compound lenses to be employed as simple microscopes, explained in a previous chapter.