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
