d a _camera
obscura_, but when the eye is used and sees the inverted image in the air,
then the apparatus is termed a _telescope_.

The image formed by a convex lens can be regarded as a new object, and if a
second lens is placed behind it a second image will be formed in the same
manner as if the first image were a real object. A succession of images can
thus be formed by convex lenses, the last image being always treated as a
fresh object, and being always an inverted image of the one before. From
this it will be evident that additional magnifying power can be given to
our telescope with one lens by bringing the image nearer the eye, and this
is accomplished by placing a short focus lens between the image and the
eye. By using a lens having a focal length of 1 inch, and such a lens will
magnify 6 times, the total magnifying power of the two lenses will be 10 x
6 = 60 times, or 10 times by the first lens and 6 times by the second. Such
an instrument is known as a _compound or astronomical telescope_, and the
first lens is called the object glass and the second lens the magnifying
glass, or eye-piece.

We are now in a position to understand how virtual images are formed, and
the formation of a virtual image by means of a convex lens will be readily
followed from a {137} study of Fig. 73. Let L represent a double convex
lens, with an object, AB, placed between it and the point F, which is the
principal focus of the lens. The rays from the object AB are refracted on
passing through the lens, and again refracted on leaving the lens, so that
an image of the object is formed at the eye, N. As it is impossible for the
eye to follow the bent rays from the object, a virtual image is formed and
is seen at A^1B^1, and is really a continuation of the emergent rays. The
magnifying power of such a lens may be found by dividing 6 inches by the
focal length of the lens, 6 inches being the distance at which we see small
objects most distinctly. A lens having a focal length of 1/4 inch would
magnify 24 times, and one with a focal length of 1/100th of an inch 600
times, and so on. The magnifying power is greater as the lens is more
convex and the object near to the principal focus. When a single lens is
applied in this manner it is termed a _single microscope_, but when more
than one lens is employed in order to increase the magnifying power, as in
the telescope, then the apparatus is termed a _compound microscope_.

[Illustration: FIG.