If a position circle be added, the instrument is called The Position Micrometer. The box containing the parallel spider’s threads and another single thread at right angles to them is attached to a move-able graduated circle. Suppose it be desired to measure the angle and distance apart of a double star, the observer should, as a rule, proceed thus :—Turn the circle till the single thread bisects one of the stars during its passage across the field of view, and make the circle to read at zero. The single line should now be brought to bisect each star, and the reading of the graduated circle will show the position angle of the stars. Their distance apart may be obtained by means of the parallel wires or threads, a wire being brought over each star as before described.

As the spider threads, from their excessive fineness, will be invisible in the field of view, some illumination of them is necessary, and this is generally effected by means of a small ray of light from a lamp.

THE DOUBLE IMAGE MICROMETER.

This instrument generally consists of a lens which has been divided in two semicircles, which can be separated by means of graduated heads, each half then forming a picture of the object viewed. Its method of employment will be thus :—Suppose, for example, it be desired to measure the diameter of the moon, the discs would be separated until the edge of the right limb of one image touches the left limb of the other; the number of revolutions of the graduated heads which have been necessary to do this reduced to their value in arc will give the lunar diameter at the time of measurement. The advantage of this form of the instrument consists principally in its applicability to other telescopes than those driven by clockwork. Other forms of micrometers have been invented, but those just described are in most constant employment.

BERTHON’S DYNAMOMETER.

By the aid of this simple but effective little instrument, the magnifying power of any eye-piece can be accurately ascertained. It can also be used to measure the diameter of wire, the thickness of metal, or indeed any round or flat object whose diameter or thickness does not exceed two-tenths of an inch; and its accuracy is such that its indications are correct to the one-thousandth part of an inch.

If a telescope, when focussed as for viewing a star, is directed to a bright part of the sky, or, better still, where the diameter of the telescope is large, to a distant and moderately well lighted object, and the eye is placed about io inches directly behind the eye-piece, a small clearly defined circle will be seen immediately in front of the eye-lens. This small disc is the miniature image of the object-glass

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