Page:Astronomy for Everybody.djvu/82

declination axis. It passes through a sheath fixed to the upper end of the polar axis so as to form a cross like the letter T. By turning the telescope on the two axes, it can be pointed wherever we choose.

Owing to the polar axis being parallel to that of the earth, its inclination to the horizon is equal to the latitude of the place. In our latitudes, especially in the southern portions of the United States, it will be nearer horizontal than vertical. But in the observatories of northern Europe, it is more nearly vertical.

It will be seen that the contrivance we have described does not solve the problem of bringing a star into the field of view of the telescope, or as we commonly say, of finding it. We might grope round for minutes or even hours without succeeding in this. There are two processes by which a star may be found:

Every telescope for astronomical purposes is supplied with a smaller telescope fastened to the lower end of its tube, and called the finder. This finder is of low magnifying power, and therefore has a large field of view. By sighting along the outside of it, the observer, if he can see the star, can point the finder at it so nearly that it wdll be in the field of view of the latter. Having found it there, he moves the telescope so that the object shall be seen in the centre of the field. Having brought it there, it is in the field of view of the main telescope.

But most of the objects which the astronomer has to observe are totally invisible to the naked eye. He must, therefore, have a system by which a telescope can be pointed at a star, without any attempt on his part to see