In our chapters, dealing with, the history of the Microscope, we attempted to trace the gradual development of the compound instrument from the simple lens; we stated that the latter, in a crude form, had been known and used from very early times and that the former developed side by side with the telescope. We have also said a few words in Chapter III. concerning light for the reason that the microscope can be better understood and used more efficiently when we are acquainted with the phenomena due to light. The simple lens, sold under the name of pocket magnifier, in its cheapest form consists of a double convex lens, that is to say, a lens with two outwardly curved surfaces. Better quality pocket magnifiers consist of two or more lenses, which may be either double convex; plano-convex, _i.e._, with one surface perfectly flat and the other outwardly curved, or they may be constructed of a combination of double convex and plano-concave lenses, such as were described on p. . The object of both the simple and compound microscope is to make objects appear larger than they do to the naked eye. When we buy our pocket lens we shall find that these little instruments are constructed to give different degrees of enlargement, some make objects appear five times larger than they do to the naked eye, some ten, some fifteen and some twenty times larger. Twenty times is about the limit of magnification for the ordinary pocket lens. If we are observant we shall notice something else--the greater the magnification the nearer we must hold the lens to our object. Within certain limits, this is not a very serious matter, but a point is reached where we must hold our lens so near to the object that we cannot see it, and that is why we cannot obtain very great enlargement with a pocket lens. Despite this fact, as we read in our opening chapter, some very wonderful discoveries have been made with these simple microscopes. Now we wish to show how a compound microscope works and, having done so, to explain the uses of its various parts. We shall consider the lenses of the instrument to be double convex; we do this for the sake of simplicity. Even in the cheapest compound microscopes of to-day simple convex lenses are never used, for the reason we explained in our last chapter. To understand the course of the light rays passing through our microscope, however, we may look upon the lenses as being merely double convex.