Having laid before the reader the principal improvements that have been suggested, or made in the single microscope, it remains only to point out those instruments of this kind, which, from the mode in which they are fitted up, seem best adapted for general use; the peculiar advantages of which, as well as the manner of using them, will be described in the third chapter of this work. Fig. 1. Plate VI. A botanical microscope, contrived by Dr. Withering. Fig. 2. Plate VI. A botanical microscope, by Mr. B. Martin, being the most universal pocket microscope. Fig. 3. Plate VI, represents that which was used by M. Lyonnet for dissecting the cossus. Fig. 5. Plate VI. The tooth and pinion microscope, which is now generally substituted in the room of Wilson’s. Fig. 1. Plate II. B. Fig. 1. Plate VII. B. The aquatic microscope used by Mr. Ellis for investigating the nature of coralline, and recommended to botanists by Mr. Curtis, in his valuable publication, the “Flora Londinensis.” Fig. 7. Plate VIII. A botanical magnifier, or hand megalascope, which by the different combinations of its three lenses produces seven different magnifying powers; when the three are used together, they are turned in, and the object viewed through the apertures in the sides. Fig. 8. Plate VIII. A botanical magnifier, having one large lens and two small ones, but not admitting of more than three powers. A COMPOUND MICROSCOPE, as it consists of two, three, or more glasses, is more easily varied, and is susceptible of greater changes in its construction, than the single microscope. The number of the lenses, of which it is formed, may be increased or diminished, their respective positions may be varied, and the form in which they are mounted be altered almost ad infinitum. But among these varieties, some will be found more deserving of attention than others; we shall here treat of these only. The three first compound microscopes deserving of notice, are those of Dr. Hooke, Eustachio Divinis, and Philip Bonnani. Dr. Hooke gives an account of his in the preface to his Micrographia, which has been already cited; it was about three inches in diameter, seven long, and furnished with four draw-out tubes, by which it might be lengthened as occasion required: it had three glasses--a small object glass, a middle glass, and a deep eye glass. Dr. Hooke used all the glasses when he wanted to take in a considerable part of an object at once, as by the middle glass a number of radiating pencils were conveyed to the eye, which would otherwise have been lost: but when he wanted to examine with accuracy the small parts of any substance, he took out the middle glass, and only made use of the eye and object lenses; for the fewer the refractions are, the clearer and more bright the object appears.