When speaking of convex lenses, we described the properties of the ordinary magnifying-glass, or simple microscope. The uses of this instrument are almost too well known to need description. It is used by old people, the lenses of whose eyes have become flattened by old age, by watchmakers for examining the minute portions of their work, by jewellers for the same purpose, and by most people for examining maps, engravings, and photographs. Simple microscopes are generally mounted in horn, ivory, or metal handles for convenience’ sake. Some simple microscopes consist of two or more lenses mounted together in order to increase the magnifying power. The student must distinguish between several lenses mounted together in this way, and the true compound microscope, which is a comparatively complicated optical arrangement, as we shall see presently. When two single lenses are thus mounted together, the power of the combination is equal to the powers of each added together. There is good reason for supposing that the simple microscope is a comparatively ancient invention. Seneca, who lived in the first century, declares that in his time it was well known that, when writing was looked at through a globe full of water, it appeared larger and blacker. In the eighth century we find the use of magnifying spectacles for old people common in most countries, and yet it was only at the beginning of the seventeenth century that a true optical instrument, in the form of a telescope, was invented. It only needed the placing of two magnifying glasses in a line to discover the principle of the telescope, but nearly a thousand years elapsed after the first introduction of these glasses before an accident rendered the principle evident. In fig. 37 we see the commonest form of microscope in the hands of an observer; and by examining the following figure and tracing out the path of the rays, we shall easily discover the principles on which its action depends. [Illustration: FIG. 37.—The Compound Microscope. ]