1 The eye should be protected from all extraneous light, and should not receive any of the light which prticeeds from the illuminating centre, excepting that portion of it which is transmitted through, or reflected from the object.
2. Delicate microscopical observations should not be made when the fluid which lubricates the cornea of the observer's eye happens to be in a viscid state, which is frequently the case. See Brande's Journal, vol. ii. p. 127.
3. The figure of the cornea will he least injured by the lubricating fluid, either by collecting over any part of the cornea, or moving over it, when the observer is lying on his back, or standing vertically. When he is looking downwards, as into the compound vertical mi croscope, the fluid has a tendency to flow towards the pupil, and injure the distinctness of the vision.
4. If the microscopic object is longitudinal, like a fine hair, or consists of longitudinal stripes, the direc tion of the lines or stripes should be towards the ob server's body, in order that their form may be least in jured by the descent of the lubricating fluid over the cornea.
5 The field of view should be contracted so as to exclude every part of the object excepting that which is under immediate examination.
6. The light which is employed for the purpose of illuminating the object should have as small a diameter as possible. In the clay time it should be a single hole in the window-shutter of a darkened room, and at night it should be an aperture placed before an Argand lamp.
7. In all cases, and particularly when very high powers are requisite, the natural diameter of the light employed should he diminished, and its intensity in creased by optical contrivances.
8. When a strong light can be obtained, and indeed in almost every case,homogeneouslight should be thrown upon the object. This may be done, either by decom posing the light with a prism, or by transmitting it through a coloured glass, which has the property of admitting only homogeneous rays.
On Solar Microscopes.
The solar microscope is an instrument for represent ing, upon the wall of a dark room, magnified repre sentations of minute objects, illuminated by the con densed tight of the sun. It was imemed, in the year 1738, by Dr. Lieberkhun, who, in the winter of 1739, when he was in London, showed one constructed by himself, to several members of the Royal Society, and several opticians, particularly Mr. Cull* and Mi. Adams. Lieberkhun's solar microscope had no mirror fen re flecting the sun's rays into the tube ; hut Mr. Cuff soon
saw its imperfections, and constructed one in a very improved form.
Mr. Cuff's solar microscope was composed of a tube, a looking-glass, a convex lens, and a Wilson's micro scope. The sun's rays were directed by the looking glass through the tube upon the object, placed a little belore the anterior focus of the convex glass. The image of the object was thrown upon a screen of white paper, and its magnitude was proportional to the dis tance of the screen from the convex M Lie berkhun afterwards adapted the solar microscope to the representation of opaque objects ; but he did not leave behind him any account of the method which he followed.
M. £pinus was the first person who described an apparatus for illuminating opaque objects in the solar microscope. If we suppose ef; Plate CCCLXXVII. Fig 25. to be the object placed before the convex lens K, he attached to the tube MNOP, two parallel brass plates AB, AC, one of which, AB, moved round a joint at A, and could be placed at any angle with AC by means of the screw Cn and spring s. On the lower end of AB and below K. he fixed a mirror 6 d, which received the rays a c from the illuminating glass NP, and threw them upon the front of the object e f By turning the screw C n, these rays could be reflected at pleasure upon any part of the object, See Nov. Comm. Petrop. vol. ix. p 326.
The solar microscope• received great improvements from Mr Benjamin M min, who has given an account of them, in his Description and use of an Opaque Solar Mirroscope. 8vo. 1774 This instrument is represented in Plate CCCLXXVII Fig. 26-33. with all the parts which are used both Inr transparent and opaque- objects. In Fig. 26. it is shewn as fitted up for opaque objects. In. Fig. 27. is represented that part of it, called the single tooth and pinion microscope, which is used for sheaving transparent objects. The cylindrical tube Y slides in the tube EF, Fig 26. and in order to use it as a single microscope, a handle c screws into a female screw at g, and the tube Y is removed. The slider shewn in Fig. 28. and containing six magnifiers, fits into a dovetail circle P, Fig. 27. The slider at Fig. 29. slides in at h, Fig. 27. and is used to condense the sun's rays strongly on the object. Three of them marked 1, 2, 3, are used, corresponding to the numbers in Fig. 28.