MICROSCOPE; from two Greek words, signifying small, and to view. An optical instrument for viewing small objects, rendering those visible which can not 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, confound-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 dis tance 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 bring 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 on; loy 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 rays 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 micro scope is dependant on the natufe 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 • at d there is attached an arm e; thisarm, being jointed at d, is capable of lying fiat, 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.
if 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-glass, a magnified representation of the en larged image is obtained. The dis tance 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 micro scopes 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, howeveri 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 power. For these purposes, there is usually introduced a third, or field by which the extent of view is still further increased by the rays being bent this lens, so tbak a greater portion of them may be refracted by the eye glass. 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 o; by this lens, an enlar and inverted image is form 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 &greater portion of them is refracted by the eye-glass c.