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The lenses and mirrors whose properties we have been considering in the previous chapters, have been combined in different ways for the purpose of examining objects too small or too distant to be perceived by the human eye. To instruments used for the former purpose the name of microscope has been given, from two Greek words signifying small and to see. In like manner the name of telescope is also derived from two Greek words, meaning distant and to see. Besides these two classes of optical instruments, others have been devised to facilitate the depicting of natural objects, either by means of the pencil or of photography, or to amuse the eye by optical illusions. Thus we have the camera obscura, the camera lucida, the magic lantern, the phantasmagoria, and numberless other instruments of the same sort, most of which will be described in the latter part of this book.

There are two sorts of microscopes, the simple and the compound; the one consisting of a single convex lens, and the other of several combinations of both convex and concave lenses.

When speaking of convex lenses, we described the properties of the ordinary magnifying glass, or simple