This microscope is evidently one which operates both by reflection and refraction. The anterior part is a con cave mirror, by which all objects placed between its surface and its focus are magnified; and, as Dr. Smith has shewn, it magnifies objects 3} times more than if they were presented to it in the usual manner. See Smith's Optics, Vol. II. p. 395.
Mr. Gray suggested also a single reflecting micros cope, founded on a similar principle. Let A, (Plate CCCLXXVII. Fig. 4.) represent a small flat ring of brass, whose mean diameter does not exceed of an inch, and which is of an inch thick. Take a small globule of quicksilver, and dissolve it in a few drops of aquafortis, to which add 10 parts of common water ; dip the end of a stick in this liquor, and rub with it the inner circle of the ring A, so as to give it a mercurial tincture. After it is wiped dry, and laid upon the table, pour a drop of quicksilver within it, which be ing pressed gently with the ball of the finger, will adhere to the ring ; and when it is cleansed with a hare's foot, it will farm a convex speculum. Take up the ring and speculum, carrying it horizontal, and lay it on the brims of the hollow cylinder B, and the mercury will now be come a concave reflecting speculum, which may be used as a reflecting microscope. Phil. Trans. 1697 , No. 228, p. 539.
Single microscopes, of a very simple kind, were con structed by Dr. Brewster, by taking up drops of very pure turpentine varnish, and allowing them to fall on a plate of thin and parallel glass. In this way he formed plano-convex lenses of any focal length ; and, by dropping the varnish on both sides, he formed dou ble convex lenses, with their convexities in any requi red ratio. The focal length of these lenses increase a little after they are formed, on the upper side of the glass, but diminish if they are formed on the lower side of it ; and if they are preserved from dust, they will last a long time. Dr. Brewster employed these fluid lenses as the object glasses of compound microscopes.
A very ingenious method of forming single micro scopes of glass has been recently proposed and executed 1)y Mr. Sivright of I\Ieggetland. 'lake a piece of pla tinum leaf, about the thickness of tinfoil, and make two or three circular holes in it, from one-twentieth to one-tenth of an inch in diameter, and at the distance of about half an inch from each other. In the holes put pieces of glass, which will stick in them without fall ing through, and which are thick enough to fill the apertures. \Vhen the glass is melted at the flame of a candle with the blow-pipe, it forms a lens which ad heres strongly to the metal, and the lens is therefore formed and set at the same time. The pieces of glass used for this purpose should have no mark of a dia mond or file upon them, as the mark always remains, however strongly they are heated with the blow-pipe. The lenses which were made larger than one-tenth of an inch were not so good as the rest, and the best were even of a smaller size than one-tenth. As the lenses thus formed sometimes contain air bubbles, the best way is to make several, and to select those which are freest from imperfections. An eye or loop, made by
bending the extremity of a platinum wire, may be used instead of the platinum leaf. The reason of using pla tinum is, that the glass is more easily and more per fectly melted in this than in other metals, which may perhaps arise from its being a bad conductor of heat, and from its preserving its brightness. As platinum does not oxidate, the glass adheres better to the edges of the holes, and it may be used very thin, as it does not melt with the heat necessary for the complete fu sion of the glass.
Mr. Sivright has likewise succeeded in forming plano-convex lenses by means of fusion. Having laid a fragment of glass upon a flat and perfectly polished natural surface of topaz, which is easily obtained by cleavage, he exposes the whole to an intense heat. The upper surface of the glass . assumes a spherical figure, in virtue of the mutual attraction of its parts, and the lower surface becomes perfectly flat, and highly polished, from its contact with the polished plate of to paz. See Edinburgh Philosoph. Journal, vol. I. p. 81.
A new kind of single microscopes, which possess particular properties, have been lately constructed by Dr. Brewster. They consist of plano-convex or dou ble-convex lenses of a hemispherical or spherical form, which may be converted into single microscopes, hav ing a much greater magnifying power than when they are used in the ordinary way. Let ABC, (Plate CCCLXXVII. Fig. 5.) for example, be a hemispherical plano-convex lens, which may be used as a single micro scope, either by presenting the side A or the side D to the object. But if we place the object at mn, and look ing in at F, examine it after reflexion from the surface BC, we shall have an effect the same as if we had placed the two lenses A a B d, A a c d, with their plane sides AB, AC together, or rather of a double-convex lens si milar to the two united. As the light is incident on BDC at an angle of it will suffer total reflexion, and not a ray of it will be lost. The spherical aberration of the lens used in this way, is to its spherical aberration used in the common way, as 97 to 117. A hemispherical lens employed in this !milliner, and having the line BC inclined to the axis of telescope, will form the best possible diagonal eye-piece. When the segment ABC is less or greater than a hemisphere, the same effects will be produced. With a double-convex lens, as shewn in Plate CCCLXXVII. Fig. 6. a very high degree of magnifying power is obtained by looking in at E, and we have the effect of a compound microscope, consisting of an object-lens AC, a concave mirror CBD, and an eye glass AC. The perfection of the image will, in this case, increase, as the incident or reflected rays approach to DA. In these, as well as in all other microscopes, the object should be illuminated from one point, such as a single candle or luminous aperture. See the Edinburgh Philosophical Journal, No. V.