From two Greek words, signifying small, and to view. An optical instrument for viewing small objects, rendering those visible which cannot be distinguished by the naked eye, and magnifying those that can. The forms of microscopes are very numerous, but they may all be included in three distinct classes, namely, single, compound-refracting, and compound-reflecting microscopes. A simple, or single microscope, is that which consists of a single lens or single spherule. Most persons may have observed, that when the distance of an object is decreased, we are enabled to define its parts more readily, and that it appears larger; thus, if we look at two men, the one at 200 feet, and the other at only 100 feet from us, the former will appear only half the height of the latter; or the angle which the latter forms with the eye of the observer will be twice that of the former. Hence the nearer we can bring an object to the eye, the larger it will appear. If we have to examine a very minute object, and in order to render its parts distinguishable, if we brine it very near to the eye, (suppose one or two inches,) it will become very indistinct and confused.

This effect is produced by the great divergency of the rays of light from the object, and the power of the crystalline lens of the eye not being sufficient to collect the rays, whereby an image of the object may be formed on the retina, at the proper distance at the back of the eye. But if we employ a single microscope, which consists of a convex lens, usually made of glass, (though it would have the power of magnifying or increasing the angle, if made of any other transparent substance, but in a different degree,) mounted or fixed in brass, and place it between the object and the eye, the former being in the focus of the glass, the diverging tays from the object will be refracted and rendered parallel by the lens, and we shall thus obtain a near and distinct view of the object. The quantity of light necessary to be employed in using a microscope is dependant on the nature of the object under examination, and on the magnifying power of the lenses necessary for its developement.

The annexed figure is a single microscope; a is the brass stem; c the cell containing the lens c; at d there is attached an arm e; this arm, being jointed at d, is capable of lying flat, or being altered to any convenient position for viewing the object, as shown by the dotted lines; on the round arm e is a sliding tube f, fixed to another tube at right angles, which carries the forceps h, movable in every direction with respect to the lens c; the handle i is screwed to the stem a when in use. This is the most convenient form of a single microscope.

A compound refracting Microscope is an instrument consisting of two or more convex lenses, by one of which an enlarged image of the object is formed, and then by means of the other, employed as an eye-glas3, a magnified representation of the enlarged image is obtained. The distance at which the two lenses of a compound microscope are placed from each other must always exceed the sum of their focal lengths, in order that the image may be formed by the object-glass in the exterior focus of the eye-glass. The great distinction between single and compound microscopes is, that in the latter we only view a magnified image of the object, while in the former we see the object itself. From this it must be evident, that unless the image formed by the object-glass be a perfect representation of the object in every particular, its imperfections, however small, will be increased by the eye-glass, in the same ratio as it magnifies the image. On account of this disadvantage, the compound microscope had been entirely laid aside by the most distinguished naturalists and philosophers till very lately. For general purposes it is prefered, on account of the extent of field obtained by it, which is far greater than that obtained by ordinary single glass lenses of equal power.

For these purposes, there is usually introduced a third, or fieldglass, by which the extent of view is still farther increased by the rays being bent by this lens, so that a greater portion of them may be refracted by the eveglass. The annexed figure is a section of a compound microscope; r is the object intended to be magnified, which is placed in the focus of the object-glass o; by this lens, an enlarged and inverted image is formed at i i, in the focus of the eye-glass c; f is a field-glass, by which the extent of the view is increased from the diverging dotted lines to i i, by the rays being bent by this lens, so that a greater portion of them is refracted by the eye-glass c.

Microscope 86

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 Obscura 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 so 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 purpose 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 eg, they are met by the magnifier n; here the rays cross, and proceed divergently to a vertical white screen prepared to receive them; 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 eg from the magnifier n; 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 magnitude 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 dimensions of an object examined with it will be magnified thirty-two times, and its surface 1024 times, or the square of 32.

Microscope 87Microscope 88

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 spherule 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-malculae 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 microscopes.

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.