The Solar Microscope consists of a common microscope, connected to a reflector and condenser, the former being used to throw the sun's light on the latter, by which it is condensed to illuminate the object placed in its focus. This microscope is sometimes called the Camera Obscure Microscope, but it still more nearly resembles the magic lantern in its effect. The exhibition it affords is made in a darkened room, and it can only be used when the sun shines. This instrument usually consists of one plane mirror and two lenses. The mirror s o must be without the window shutter d u ; the lens a b fixed in the shutter ; and the lens n within the room. The lens a b is inclosed in a brass tube, and the other in a smaller tube, which slides in the former, for the pur pose of adjusting it to the proper distance from the object. The mirror can be so turned by adjusting screws, that however obliquely the incident rays E F fall upon it, they can be reflected into the dark room through the illuminating lens a b in the shutter. This lens collects those rays into a focus near the object, and, passing on through the object cg, they are met by the magnifier n; here the rays cross, and proceed divergently to a vertical white screen prepared to receive then; on which screen, the image or shadow gr of the object will appear. The magnifying power of this instrument depends on the distance of the white screen, and in general bears a certain proportion to the distance of the object cg from the magnifier a; that is, if the screen be at ten times that distance from the lens n, the image will be ten times as long, and ten times as broad as the object. About ten or twelve feet is the best distance; for, if further off, the image, though larger, will be obscure and ill defined. The apparent magni tude of objects is measured by the angle under which they are seen by the eye, and those angles are reciprocally as the distances from the eye. If eight inches
be assumed as the nearest limit of a distinct vision to the naked eye, and by interposing a lens, we can see with equal distinctness at a nearer distance, the object will appear to be as much larger through the lens than to the naked eye, as its distance from the eye is less than the distance of unassisted vision. If the focal distance of a convex lens be one quarter of an inch, or the thirty second part of the common limit of vision, or eight inches, the lineal dimen sions of an object examined with it will be magnified thirty-two times, and its surface 1024 times, or the square of 32.
The simplest microscope which can be employed to any useful purpose, is that which is made with a drop of water, suspended in a very small hole in a thin slip of brass, or any similar material. A apherule of water, however, of the same size as one of glass, will not magnify so much as the latter, because, as its density is not so great, it has a longer focus. A drop of water placed on the end of a slender piece of brass wire, and held to the eye by candle-light, will, without any other apparatus, magnify, in a very surprising manner, the ani malculse contained in it. These water microscopes have given rise to the use of various other fluids, with several varieties of construction. Dr. Brewster, instead of water, has made use of very pure and viscid turpentine, taken up by the point of a piece of wood, and dropped successively upon a thin and well polished glass. The same gentleman has also used sulphuric acid and castor oil, both of which possess a refractive power considerably greater than water. Fluid lenses have been employed as the object-glasses of compound micro scopes. Minute glass spherules make excellent microscopes, but the foci of the smallest sort are so short, that it requires considerable attention and patience to employ them well.