A description of the microscope of Eustachio Divini, was laid before the Royal Society in 1668. It consisted of an object-glass, a middle-glass, and two eye glasses, which are plano-convex, and placed so as to touch one another in the middle of the convex surfaces. The pur pose of his construction was to shew the objects flat, and not crooked, and to take in a large field, at the same time that it had a high magnifying power. It was about 16:t inches high, and adjusted at four different lengths. In the first, which is the best, it shews lines 41 times larger than they appear to the naked eye; in the se cond, 90 times; in the third, I11 times ; and in the fourth, 143 times. The diameter of the field, or the subtense of the visual angle, measured upon the object plate, in the first length, was 8 inches 7 lines ; in the second, 12 inches 4 lines ; in the third, 13 inches; and in the fourth, a little more than 16 inches. The tube was as large as a man's leg, and the eye-glasses like the palm of the hand. See Phil. Trans. 1668. No. 42. p. 842.
Philip Bonnani, in a work entitled Observationes cir ca Viventia, que in rebus non viventibus reperiuntur, &c. 1791, published an account. of two compound micro scopes which he used. One of these was composed of an eye-glass, a middle-glass, and an object-glass. mount ed in a cylindrical tube placed horizontally. Behind the stage was a small tube, with a convex lens at each end, and a lamp beyond it, whose light was concentrated by the lenses in the tube, and thrown upon the object to be examined. The instrument possessed various adjust ments, and was regulated by a rack and pinion.
Sir Isaac Newton seems to have been the first person who suggested the use of a reflecting microscope. He placed the objects before a concave speculum, so that an enlarged image of them was formed at a greater dis tance. This image being viewed with a convex eye glass, was again magnified, and appeared very distinct. The great defect of this instrument arose from its being inapplicable to opaque objects, in consequence of the objects being placed between the object speculum and the eye-glass. See Phil. Trans. 1672, No. 10. p. 3075, and the Appendix to Gregory's Elements of Optics.
Another form of the reflecting microscope was sug gested by Dr. Rohm Barker, in the year 1736. It was nothing more than the Gregorian telescope converted into a microscope, merely by lengthing the tulle, and there fore requires no particular description. There can be no doubt that it would give very distinct images, and has the advantage of allowing the light to fall freely upon the object, which is placed at a distance of from 9 to 24 inches beyond the tube. See Phil. Trans. 1736, Vol. 39. No. 442. p. 259.
The next compound microscope was invented by Dr.
Robert Smith, Professor of Experimental philosophy at Cambridge. Having had occasion to examine the pi in ciplcs of Sir Isaac Newton's reflecting microscope, he constructed one of them, in which the focal distance of the speculum was 21 inches. lie " found that the co " lours of objects appeared much more beautiful and " natural than in doubly refracting microscopes of the " best sort, their proper colours being free from the " mixture of other colours arising in refracting micro ', scopes, from the different rclrangibility of rays."
Smith's Optics, Vol. 1. p. 279. He found also that ob jects appeared sufficiently bright, and very distinct, when the reflecting microscope had the following dimensions : Focal length of the speculum . . inches.
Diameter of the speculum . . . 1 Focal length of the plann-convex eye-glass Ratio of the distance of the object from the focus of the speculum, to the focal dis tance of the speculum . . . 1 to 14 Having found that, in order to produce a high magni fying power with this reflecting microscope, it was ne cessary to have the speculum very concave, and there fore very small, he set about contriving a microscope with two reflecting spherical surfaces of any size, so proportioned to each other that the aberration of the rays, caused by the first reflection, should be perfectly corrected by the second, and, consequently, that the last image of the object from which the rays diverge upon the eye-glass, shall be pet fectly free from aberration.
One of Dr. Smith's microscopes is shewn in Plate CCCLXXVII. Fig. 18. where AA is a concave sphe rical speculum, and CC a convex spherical speculum, having its polished convex surface inwards. The rays from an object o placed in the slider Inn, will be reflected from the concave speculum AA upon the concave CC, and will have a distinct and magnified image of it formed before the convex eye-glass E, by which it will be magnified still more. This instrument, in short, is nothing more than the Cassegrainian tele scope converted into a microscope, with this difference only, that, in the telescope, distinct vision is obtained by moving the convex mirror, whereas, in the microscope, it is obtained by a motion of the eye-glass. Dr. Smith constructed one of these microscopes, which he found to perform " nearly as well, in all respects, as the very best refracting microscopes ;" and the writer of this article has one of them now before him, which performs wonderfully well, though both the specula have their polish considerably injured. The following are the di 3ncnsions, &c. of Dr. Smith's reflecting microscope as given by himself: Focal length of both specula . . 1 0000 Distance of the centres of both specula 1.6558 Distance of the image from the centre of the concave speculum . . 1.1337 Focal length of the eye-glass . . .0.1407 Distance of the eye behind the eye glass . . . . . . 0.1479 Diameter of the eye-hole . . . 0.0190 Distance of the object from the centre of the convex speculum . . 0.0626 Length of the concave speculum . 15° 47 Arch of the convex speculum . . 50' 49" Distance of•the stop a from the object 0.4545 Diameter of the slop 0.03g Diameter of the hole in the concave spe culum . 0.143 Diameter of the hole in the convex spccu 0 049 Magnifying power, the focal length, &c. of the cyc being 8 inches . 300 times.