If we consider the construction of the different com pound microscopes which have been described, it will be obvious, that a great deal must depend on the accu racy with which the axes of the small magnifying lenses coincide with the axis of the instrument, or that of the eye-glasses. The difficulty of effecting such a coincidence is very great, and we believe is in a great measure overlooked. \Ve conceive, therefore, that the tube which holds the magnifying lenses should have an universal motion, so that, by means of screws, the axis of the lenses could be brought into perfect coin cidence wih the axis of the eye-glasses.
111. .luzici's Reflecting Microscope \Ve have already seen that the reflecting microscope of Dr. Barker was nothing more than the Gregorian telescope, with its tube lengthened so as to permit it to act as a microscope ; and that the reflecting micro scope of Dr. Smith was the Cassegrainian telescope fitted up in a similar manner. The new reflecting mi croscope constructed by Professor Amici of Modena, and with which he has made several important discoveries, is in like manner the Newtonian telescope converted in to a microscope.
It consists of a horizontal brass tube ABCD, Plate CCCLXXVII. 12 inches long, and of an inch in diameter. A concave metallic speculum AB, having its reflecting surface a portion of an ellipsoid, whose foci are F and f, is placed at one end of the tube ; the distance AF being inches, and At 12 inches. A small arm rs with in the tube, carries a plane speculum of an oval form, placed at the distance of inches from AB, support ed by an oblique section ol an metallic cylinder, of an inch in diameter, and inclined 45° to the axis If The object is placed below the tube at MN, and as far before the mirror s as the focus F of AB is behind it ; and it is supported by a moveable object-bearer attached to the pillar below it. The diameter of the tube ABCD is about 1.4 inches, and the thickness of its metal Ath. The object is always placed at the distance of hall an inch from the edge of the tube, and may therefore be illuminated by a convex lens, attached in the usual way to the stage or object-bearer. A large concave illuminating mirror below for throwing light upon transparent obji cts, has a diameter of about 3 inches, and a focal length of abou' M. Amici has been able to apply to this nt a power of one mil lion*, calculating the number of times that the superficies is magnified, whereas the best microscopes of Adams; Dollond, Delebarre, do not in.gnify more than 225,d10,
or about 475 times in diameter. See The Edinburgh Philosophical Journal, Vol. i. p. 135, where a more detailed account of this instrument will be found.
IV. Dr. Brewster's Reflecting Microscope.
This instrument, which will be understood from Plate CCCLXX VII. Fig. 22, possesses several advantages, and in particular that of being applicable to the stand and apparatus of any compound microscope. Though it is drawn in a horizontal position, it is intended to be placed vertically. It consists of a concave speculum AB, whose surface is a portion of au ellipsoid, having F andf for its foci. It is perforated at 0, like the speculum of a Gre gorian telescope ; and between this aperture and F is placed a small convex speculum s, whose surface is part of an ellipsoid, having F and Q for its foci, a rp be ing equal to MN, the distance of the object. The back ol the speculum is ground into a concave form a b, for the purpose of condensing the light upon the ob ject MN. The rays diverging from MN are made to diverge still more by reflection from the small mirror s, so that F is their virtual focus ; and these reflected rays, falling upon the speculum AB, as if they had di verged from one of its foci F, will be reflected to f, and there form a magnified image, which will be again magnified by the eye-glass placed at LL. The convex mirror s is carried by the arm 8 r, which is moved by means of the screw r T, so that F is one of its foci. An object placed at MN will therefore be seen very distinctly by a microscope of this kind ; and it has such a distance from the mirror AB, as to be capable of receiving any degree of artificial illumination. if this microscope is placed vertically in the arm ED of the compound microscope shown in Fig. 19, made large enough to receive it, it may be used along with all the apparatus adapted to that or to any other com pound microscope. if a change of magnifying power is required, we have only to use a deeper eye-glass LL ; and as the place of the object is not altered by this change, the concave illuminating speculum a b will still condense upon the object MN, the light re flected from the great mirror U, Fig. 19.