Having arrived at the fundamental principle that part of a lens will act in a manner similar to a whole one, it is easy to explain the construction of a heliometer. An ordinary telescope lens is sawed in half by means of a thin round metal disk revolved rapidly by machinery, and fed continually with emery and water at its edge. The cutting effect of emery is sufficient to make such a disk enter glass much as an ordinary saw penetrates wood. The two "semi-lenses," as they are called, are then mounted separately in metal holders. These are attached to one end of the heliometer, called the "head," in such a way that the two semi-lenses can slide side by side upon metal guides. This head is then fastened to one end of a telescope tube mounted in the usual way. The "head" end of the instrument is turned toward the sky in observing, and at the eye-end is placed the usual little magnifier we have already described. The observer at the eye-end has control of certain rods by means of which he can push the semi-lenses on their slides in the head at the other end of the tube. Now, if he moves the semi-lenses so as to bring them side by side exactly, the whole arrangement will act like an ordinary telescope. For the semi-lenses will then fit together just as if the original glass had never been cut. But if the half-lenses are separated a little on their slides, each will act by itself. Being slightly separated, each will cover a different part of the sky. The whole affair acts as if the observer at the eye-end were looking through two telescopes at once. For each semi-lens acts independently, just as if it were a complete glass of only half the size. Now, suppose there were a couple of stars in the sky, one in the part covered by the first semi-lens, and one in the part covered by the second. The observer would, of course, see both stars at once upon looking into the little magnifier at the eye-end of the heliometer.