The compound microscope may, consist of only two lenses, while simple microscope has been shown to contain sometimes three. In the triple for the simple microscope, however, it was explained that the object of the first two lenses was to do what might have been accomplished, though not so well, by one; and the third merely reflected certain modifications in the light before it entered the eye.

But in the compound microscope the two lenses have totally different functions: the first receives the rays from the object, and bringing them to new foci, forms an image, which the second lens treats as an original object, and magnifies it just as the single microscope magnified the object itself.

It shows the earliest form of the compound microscope, with the magnified image of a fly, as given by Adams, which he describes as consisting of an object-glass, a field glass, and an eye-glass; the object, being placed a little further from the lens than its principal focal distance, the pencil of rays from which converge to a focus, and form an inverted image of the object, which image is viewed by the eye placed at a through the eye-glass.

The rays remain parallel after passing out until they reach the eye, when they will converge by the refractive powers of this organ, and be collected on the retina. But the image differs from the real object in a very essential particular. The light being emitted from the object in every direction, renders it visible to an eye placed in any position; but the points of the image formed by a lens emitting no more than a small conical body of rays, which is receives from the glass, can be visible only to the eye situate within its range.

Thus the pencil of rays emanating from the object, unless converged by the field-lens, would cross each other, and diverge and therefore would never arrive at the len, without the interposition of the plano-convex lens. The object is magnified upon two accounts: first, because if we view the image with the naked eye, it would appear as much longer than the object as the image is really longer than it, and secondly, because this picture is again magnified by the eye-glass.

The compound microscope, then, consists of an object-lens, by which the image is formed, enlarged, and inverted; am amplifying lens, by which the field of view is enlarged, and is consequently called the field-glass; and an eye-glass or lens, by which the eye is permitted to approach very near, and consequently enabled to view the image under a large angle apparent magnitude. The two, when combined, are termed the eye-piece.