Spherometer

Spherometer consists of a circular disc of metal with a graduated edge. This rests upon three equal equidistant legs whose points are hard and rounded. A screw is fixed to the centre of the disc, and its end is also hard and rounded. When the spherometer rests upon a plane - say, a smooth sheet of brass - it is perfectly steady on its three feet; but the disc may be turned till the fourth foot also meets the glass. At one point all four feet are in a plane, but another fraction of a turn brings the fourth foot too far down, and the whole instrument rocks. The point when rocking is just about to commence is the point when the fourth foot is exactly level with the other three. If the instrument be now transferred to a lens, the central foot must be screwed up to let the three feet take a steady position, and then screwed down till rocking is again just about to commence. The distance through which the foot has been moved since it was on the flat plate is read off on a fixed upright scale, and fractions of a revolution are obtained by observing the position of the graduated disc. The curvature of the lens can then be quite simply deduced.