Fig. 52.—Simple Microscope.
When the magnifying power of a lens is considerable, or when its focal length is short, or it is wished to use it with greater precision and steadiness, it should be mounted on a short stand with a tubular stem, with rack-work focussing movement and mirror illumination. [Fig. 52] represents a simple dissecting microscope, with a glass circular stage, 4½ inches in diameter, supported on three legs—a handy and useful form of instrument for many purposes.
The Compound Microscope.
The compound microscope differs from the simple, inasmuch as the image is formed by an object-glass, and further magnified by one or more lenses forming an eye-glass. For a microscope to be a compound one, its essential qualification is that it should have an object-glass or objective, and an eye-glass or eye-piece, so called because they are respectively near the object and the eye of the observer when the instrument is in use. The microscope consists of a tube or body, and a stand, an arrangement for carrying the body, combined with which is a stage for holding the object, and a mirror for its illumination. To the more modern instrument has been added a substage, to carry a condenser and other accessories.
The body of a microscope, which carries the system of magnifying lenses, must be placed at one particular distance from the object, termed the focus, in order that a clear image may be obtained. For the purpose of focussing two motions are supplied, the one for coarse adjustment, with lower powers; the other for higher powers, termed the fine adjustment. It is in this wise that the magnifying power of the compound microscope is turned to good account.
There are, however, limits to the use to which lenses can be put with advantage in the direction of magnifying the object, just as there are in varying the magnifying power of the eye-glass. Defects in either, although not first seen, that is, when the image is but moderately enlarged, are brought into prominence by greater amplification. In practice, therefore, it is found to be of advantage to vary the power by employing object-glasses of different values (foci). In whatever way increase of amplification is brought about, two things will always result from the change: the proportion of surface of the object of which an image can be formed must be diminished, and the amount of light spread over the image proportionally lessened.
In addition to the two lenses mentioned, it was found to be of considerable advantage to introduce a third lens between the object-glass and the image formed by it at eye-piece, the purport of which is to change the course of the rays (bend in the pencil) so that the image may not be found of too great a dimension for the whole to be brought within the circumference of the eye-glass. This, it will be readily seen, allows more of the object to be viewed at the same time by the field-glass, as the eye-piece of the microscope is termed.
Fig. 53.
[Fig. 53] represents the body of an ordinary compound microscope with its triplet object-glasses; o is an object, above it is the triple achromatic object-glass, in connection with the eye-piece e e, f f the plano-convex lenses; e e being the eye-glass, and f f the field-glass, between which, at b b, the arrow represents the diaphragm. The course of the light is shown by three rays drawn from the centre, and three from each end of the object o; these rays, if not prevented by the lens f f, and the diaphragm b b, would form an image at a a; but here, as they meet with the lens f f in their passage, are converged by it at b b, the diaphragm at b b intercepting a portion of peripheral rays, permitting only those to pass that are necessary for the formation of the image, the further magnification of which is, however, here brought about by the eye-glass e e, precisely as if it were that of the original object under examination. It will be apparent, then, that the field-lens f f belongs in principle to the object-glass, or objective, taking a share in the image-forming rays, although this is taken to be a part of the eye-piece.