On the other hand, to bring an object into the field of view of an alt-azimuth, it is necessary, either that the object itself should be visible to the naked eye, or else that the position of the object should be pretty accurately learned from star-maps, so that it may be picked up by the alt-azimuth after a little searching.

A small telescope called a _finder_ is usually attached to all powerful telescopes intended for general observation.

The finder has a large field of view, and is adjusted so as to have its axis parallel to that of the large telescope.
Thus a star brought to the centre of the large field of the finder (indicated by the intersection of two lines placed at the focus of the eye-glass) is at, or very near, the centre of the small field of the large telescope.
If a telescope has no finder, it will be easy for the student to construct one for himself, and will be a useful exercise in optics.

Two convex lenses not very different in size from those shown in fig.

1, and placed as there shown--the distance between them being the sum of the focal lengths of the two glasses--in a small tube of card, wood, or tin, will serve the purpose of a finder for a small telescope.