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. [Illustration: Fig. 53.]