the finder, a small telescope of low power and large field, used for finding celestial objects not easily visible to the naked eye. It is so arranged, that when the object is found and carried to its centre, it is also in the centre of the field of the larger instrument. The handle and the two toothed wheels serve to raise or lower the telescope, which is movable on the horizontal axis, which supports it in front, so that it may be directed to any part of the heavens the observer may desire.
The following figure shows the arrangement of the lenses, and the path of the rays through them, in telescopes of this form.
Fig. 43.—Section of an Astronomical Telescope.
The convex lens which serves as an object-glass, gives at a b a reversed image of the star A B. The small convex lens which acts the part of an eye-piece, enlarged this reversed image without changing its position, and causes it to be seen in the line A' B'. This eye-piece is fixed at the extremity of a tube, which is smaller than that containing the object-glass, and slides easily backwards and forwards from the spot where the image a b is found. The latter is an indispensable condition, for it is rare to meet two persons whose eyes are of the same focus; besides, the image a b will fall at a different
