2. THE NECESSITY OF THE MICROSCOPE.--If, therefore, we would know the real structure of the human body, we must make use of the microscope. We are not at liberty either to use it or not; we _must_ have recourse to it in order to obtain a real knowledge of the human body. Our eyes are constructed for the common offices of life, to provide for our wants and guard us from the ordinary sources of danger; but by arming them with _lenses_, the real structure of plants and animals is revealed to our intelligence; and enemies, otherwise invisible, that lie in wait in the air we breathe, and in our daily food and drink, to destroy life, are guarded against. [Sidenote: 3. What are convex lenses? Kind of lenses used in microscopes? Experiment? Picture thrown upon the eye? Derivation of the word microscope?] 3. CONVEX LENSES, or magnifying glasses, are disks of glass or other transparent substance, which have the {237} property of picturing upon the retina of the eye an image of an object larger than the image produced there without their aid. The glasses used in microscopes are either double convex lenses (_a_) or plano-convex lenses (_b_). If a double convex lens or a plano-convex one be placed over a hole in the shutter of a darkened room, or over the key-hole of a door, and a piece of paper be held at a proper distance, a picture of all objects in front of the lens will be thrown on the paper, as in the camera-obscura or the magic-lantern. Now, in the same manner, a lens throws a picture of objects to which it is directed on the retina of the eye, and when that picture is larger than the image made in the eye by the object, without the aid of the lens, it is magnified, or the lens has served as a _microscope_, so called, from its use in seeing small objects, from _mikros_, small, and _skopeo_, to see. [Illustration] [Sidenote: 4. Kinds of microscope? What are simple microscopes?] 4. DIFFERENT KINDS OF MICROSCOPES.--Microscopes are either _simple_ or _compound_. The glasses of magnifying spectacles, like those commonly used by aged persons, are simple microscopes. Magnifying glasses, mounted in frames such as are for sale by opticians and others, for the detection of counterfeit money, are simple microscopes, and are useful in studying the coarser structure of plants and animals. [Sidenote: 5. Construction of the most powerful simple microscopes? In practice? A doublet? Triplet? Why are compound microscopes superior to simple ones?]