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the instrument is turned on its axis, all these graduations pass successively under each microscope, so that they can be seen by the observer looking through the latter. The position of the star is determined by measures with the microscope on the graduation which happens to be under it when the telescope is pointed at a star.

The equatorial telescope and the meridian circle are the two principal instruments in the astronomical outfit of an observatory. Many other instruments are more or less in use for special purposes, but they are not of great interest, save to one who is making a special study of astronomy and who must therefore refer to books specially written for the professional student of the subject.

The precision with which a practised observer can note the time of transit of a star over the thread of his instrument is remarkable. One method of doing this consists in listening to and counting the beats of the clock as the star approaches and crosses the thread. He watches the exact position of the star at the beat before the transit, and again at the beat following. By comparing in his mind the opposite distances of the star from the thread at the two clock beats, he estimates the number of tenths of the second at which the transit took place, and records the time in his notebook.

This method is now superseded in most observatories by that of registration on a chronograph. This instrument consists of a revolving cylinder, covered with paper, having a pen-point resting upon it, so that, as the cylinder revolves, the pen leaves a trace on the paper.