Further, if we imagine a line drawn in continuation of the axis of the fork towards the object, then the angle made by this line and that from the axis of the mirror to the elbow joint of the fork (the direction of the reflected ray) will be equal to the two angles at the base of the isosceles triangle; and, since they are equal to each other, the angle made by the directing rod and the axis of the fork (or the incident ray) from the object, is equal to half the angle made by the latter ray and the direction of the reflected ray; and if lines are drawn through the surface of the mirror in continuation of the directing rod and the line from the elbow joint to the axis of the mirror; and a line to the point of intersection be drawn from the object, this last line will be parallel to the axis of the fork, and the angle it makes with the continuation of the directing rod, or normal to the surface of the mirror, will be half the angle made by it and the line representing the reflected ray. Therefore the angle made by the incident ray and the required direction of the reflected ray is always bisected by the normal, so that the reflected ray is constant in the required direction. The clock is driven in the usual manner by a weight. A rod carries the motion up to the system of wheels by which the polar axis is rotated. As this axis rotates it carries with it the fork, which transmits the required motion to the mirror. And as the fork alters its direction the tube slides upon the directing rod, thus altering the inclination of the mirror. In order to vary the position of the mirror without stopping the instrument there are slow motion rods or cords proceeding from the instrument which may be carried to any distance desirable. [Illustration: FIG. 157.—The Siderostat at Lord Lindsay’s Observatory. ]