The singular dexterity which Mr. Coventry and others now possess, of cutting by an engine fine parallel lines upon glass, pearl, ivory, and brass, at such minute distances as, by means of a microscope, are proved to be from the 100th to the 5000dth part of an inch, render this sort of micrometer the easiest and most accurate means of obtaining the exact natural size of the object to be magnified, and how many times that object is magnified. Mr. B. Martin, and other opticians, many years ago applied divided slips of glass, ivory, and horn to the body, in the focus of the eye glass of microscopes; but the thickness of the whole medium of the glass was found to diminish the distinct view of the object: ivory and horn, from their variable texture, were found to expand and contract too readily to be commodious. It is therefore to Mr. Cavallo that we are indebted for the happy thought of adapting slips of divided pearl to telescopes, to ascertain their power, &c. which substance the opticians now find to be the best for microscopical micrometers. It possesses a sufficient degree of transparency, when made about the thickness of writing paper; is a steady substance; admits very easily of the finest graduations, and is generally made in breadth about the 20th part of an inch. Fig. 9. Plate II. A, is a representation of this scale, with divisions of the 200ths of an inch, every fifth and tenth division being left longer than the others, which only go to about the middle. If the eye glass of the microscope or telescope, to which this micrometer is to be applied, magnify very much, its divisions may be proportionably minute. To measure by this micrometer the size of an object in a single microscope, nothing more is required than to lay it on the micrometer, and adjust it to the focus of the magnifier, noticing how many divisions it covers or coincides with. Supposing the parallel lines to be the 1000dths of an inch, and the object covers two divisions, its real size is the 500th of an inch; if five, 200th of an inch, &c.