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 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.