(_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. (_f_) A driving clock so connected with _b_ as to make _c_ (and _d_) revolve about _b_ with an angular velocity equal and opposite to that with which the earth turns upon its axis. [Illustration: FIG. 41.--A simple equatorial mounting.] [Illustration: FIG. 42.--Equatorial mounting of the great telescope of the Yerkes Observatory.] Such a support is called an equatorial mounting, and the student should note from the figure that the circles, _e_, measure the hour angle and declination of any star toward which the telescope is directed, and conversely if the telescope be so set that these circles indicate the hour angle and declination of any given star, the telescope will then point toward that star. In this way it is easy to find with the telescope any moderately bright star, even in broad daylight, although it is then absolutely invisible to the naked eye. The rotation of the earth about its axis will speedily carry the telescope away from the star, but if the driving clock be started, its effect is to turn the telescope toward the west just as fast as the earth's rotation carries it toward the east, and by these compensating motions to keep it directed toward the star. In Fig. 42, which represents the largest and one of the most perfect refracting telescopes ever built, let the student pick out and identify the several parts of the mounting above described. A part of the driving clock may be seen within the head of the pier. In Fig. 43 trace out the corresponding parts in the mounting of a reflecting telescope. [Illustration: FIG. 43.--The reflecting telescope of the Paris Observatory.] A telescope is often only a subordinate part of some instrument or apparatus, and then its style of mounting is determined by the requirements of the special case; but when the telescope is the chief thing, and the remainder of the apparatus is subordinate to it, the equatorial mounting is almost always adopted, although sometimes the arrangement of the parts is very different in appearance from any of those shown above. Beware of the popular error that an object held close in front of a telescope can be seen by an observer at the eyepiece. The numerous stories of astronomers who saw spiders crawling over the objective of their telescope, and imagined they were beholding strange objects in the sky, are all fictitious, since nothing on or near the objective could possibly be seen through the telescope.