5. The most powerful simple microscopes are made by melting in a flame a thread of spun glass, so as to form a {238} minute globule or bead, which, when set in a piece of metal and used to examine objects on a plate of glass held up to the light, gives a high magnifying power. In practice, however, it is found better to use several magnifying glasses of moderate power, than a simple lens alone of high power. A combination of two lenses is called a _doublet_--of three, a _triplet_. All _simple_ microscopes throw an enlarged image of the object upon the retina. _Compound_ microscopes are so constructed that the enlarged image of an object is again magnified by a second lens, and hence their magnifying power is vastly superior to that of simple microscopes. [Sidenote: 6. Explain, by means of the diagram, the action of the compound microscope.] 6. The accompanying diagrams will explain the action of the compound microscope compared with that of the simple microscope. In Fig. 58, which represents the working of the simple microscope, the rays from the object (_a b_), passing through the lens (L), form an image (_a' b'_) in the retina of the eye (E), and as all images are inverted in the eye, the object is seen as all other objects are, and appears erect. In Fig. 59 is seen the action of the compound microscope. An inverted image (_a' b'_) of the object (_a b_) is magnified by the second lens (L'), and an erect image is thrown upon the retina, which, as all other objects seen erect with the naked eye are inverted, gives to the image a contrary direction, or inverts it to the mind. [Illustration: FIG. 58.--SIMPLE MICROSCOPE.] {239} [Sidenote: 7. Portions, in a compound microscope? The glasses?] 7. A COMPOUND MICROSCOPE consists of two portions: the optical portion, or the lenses, and the mechanical portion, or the instrument which bears the lenses. The glasses of a compound microscope are two: the _object-glass_ (D), Fig. 60, and the lower lens of Fig. 59, and the _ocular_ or _eye-piece_ (A), Fig. 60, and the upper piece of Fig. 59. Both the object-glass and the eye-piece may, and usually do, consist of more than one lens, for, as previously mentioned, better results are obtained by a combination of lenses of moderate power than by single lenses of high power and great curvature. [Sidenote: 8. How to choose a microscope? How to use it?] 8. HOW TO CHOOSE AND USE A MICROSCOPE.--No attractiveness in the mechanical part of a microscope can compensate for inferior lenses; and the very first consideration in the choice of an instrument should be the excellence of the optical part of the instrument. In the use of the instrument, care should be exercised to keep the lenses clean, free from dust, not to press the object-glass upon the object under observation, and not to wet it in the water in which most objects are examined. A good microscope requires its own table; and when not in use should be covered by a bell glass or a clean linen cloth.