Make a pin hole in a piece of cardboard. Bring a printed page so close to one eye that you can no longer see the letters distinctly, and then place the pin hole between the eye and the page. The letters which were before blurred may now be seen plainly through the pin hole, even when the page is brought nearer to the eye than before. As it is brought nearer, notice how the letters seem to become larger, solely because they are nearer. A pin hole is the simplest kind of a magnifier, and the eyepiece in a telescope plays the same part as does the pin hole in the experiment; it enables the eye to be brought nearer to the image, and the shorter the focal length of the eyepiece the nearer is the eye brought to the image and the higher is the magnifying power.
Fig. 41.—A simple equatorial mounting.
80. The equatorial mounting.—Telescopes are of all sizes, from the modest opera glass which may be carried in the pocket and which requires no other support than the hand, to the giant which must have a special roof to shelter it and elaborate machinery to support and direct it toward the sky. But for even the largest telescopes this machinery consists of the following parts, which are illustrated, with exception of the last one, in the small equatorial telescope shown in [Fig. 41]. It is not customary to place a driving clock on so small a telescope as this:
(a) A supporting pier or tripod.
(b) An axis placed parallel to the axis of the earth.
(c) Another axis at right angles to b and capable of revolving upon b as an axle.
(d) The telescope tube attached to c and capable of revolving about c.
(e) Graduated circles attached to c and b to measure the amount by which the telescope is turned on these axes.