In order to do this accurately we must determine how many divisions of the screw correspond to the distance of the wires when on two stars, say, one second apart. Here we must take advantage of the rate at which a star travels across the field when the telescope is fixed, and we separate the wires by a number of turns of the screw, say twenty, and find what angle this corresponds to, by letting a star on or near the equator[11] traverse the field, and noticing the time it requires to pass from one wire to the next. Suppose it takes 26⅔ seconds, then, as fifteen seconds of arc pass over in one second of time, we must multiply 26 by 15, which gives 400, so that the distance from wire to wire is 400 seconds of arc; but this is due to twenty revolutions of the screw, so that each revolution corresponds to 400/20˝, or twenty seconds, and as each revolution is divided into 100 parts, and 20/100˝ = ⅕˝ therefore each division corresponds to ⅕˝ of arc. We shall return to the use of this most important instrument when we have described the equatorial, of which it is the constant companion. THE HELIOMETER. [Illustration: FIG. 105.—A B C. Images of Jupiter supposed to be touching; B being produced by duplication, C duplicate image on the other side of A. A B, Double Star; A, A´ & B, B´, the appearance when duplicate image is moved to the right; A´, A & B´, B, the same when moved to the left. ] [Illustration: FIG. 106.—Object-glass cut into two parts. ] [Illustration: FIG. 107.—The parts separated, and giving two images of any object. ]