The object to be looked at is placed at _a_ (fig. 38), on a piece of thin glass usually called a _slide_. A small converging lens placed at _b_ collects the rays proceeding from the object, and transmits them as far as _c d_, where they come under the influence of a second converging lens B, which causes them to spread out still more before they reach the eye. Consequently we not only see the image of the object magnified by the lens _b_, but still more enlarged by the action of the lens B, and appearing considerably enlarged at C D. The lens placed in front of the object is called the _objective_ or _object-glass_; that placed nearest the eye, the _eye-piece_. These names apply equally to the similar lenses used in telescopes and other optical instruments. The instrument shown in fig. 38 is the simplest possible compound microscope, and is very rarely used. The eye-piece is generally constructed of two lenses, and the object-glass of as many as eight; the object in multiplying the lenses being, not only to increase the magnifying power, but to decrease certain defects inherent in all lenses whose surfaces are parts of spheres. [Illustration: FIG. 38.—The Theory of the Compound Microscope. ] The amplification depends mainly upon the power of the objective, but different eye-pieces are also used to increase the apparent size of the objects to be examined. Thanks to the investigations of modern philosophers, we are enabled to magnify objects to 2,000 times their diameter with perfect distinctness; that is to say, the surface of the object appears to occupy 4,000,000 times its natural extent. Under such a power a hair would appear about six inches thick, a fine needle would look like a street post, and a grain of sand like a mass of rock. Although it is possible to employ compound microscopes of such a high magnifying power in the investigation of certain classes of objects, all ordinary preparations are best seen under a power of 500 or 600 diameters. It would be utterly impossible to give our readers the slightest idea of the benefits conferred on the human race by this marvellous instrument. Suffice it to say, that no naturalist or surgeon ever attempts the most simple investigation into the structure of any body without the aid of the microscope. It has already shown us that a world of creatures exists which, although invisible to the eye of man, are possessed of wonderful forms, colour, and beauty of structure, and is daily adding to our knowledge in this direction. We can hardly submit any substances to this marvellous instrument without discovering animal or vegetable life of the most vivid character. A drop of scum from the surface of a stagnant pool is instantly seen to be peopled with animal and vegetable life, when submitted to microscopic examination. At one moment a rolling ball glistening like glass slowly revolves past our view; then a little fellow like a piece of spiral spring screws his way