The same instrument, furnished with microscopes and accessories, may be employed for comparing lengths with a standard. It may then be called a comparator.
The spherometer is a special form of the micrometer screw. As
its name implies, it is primarily used for measuring the curvature of spherical surfaces.
It consists of a screw with a large head, divided into a great number of parts, turning in a nut supported on three legs terminating in points, which form the vertices of an equilateral triangle. The axis of revolution of the screw is perpendicular to the plane of the triangle, and passes through its centre. The screw ends in a point which may be brought into the same piano with the points of the legs. This is done by placing the legs on a truly plane surface, and turning the screw till its point is just in contact with the surface. The sense of touch will enable one to decide with great nicety when the screw is turned far enough. If, now, we note the reading of the divided scale and also that of the divided head, and then raise the screw, by turning it backward, so that the given curved surface may exactly coincide with the four points, we can compute the radius of curvature from the difference of the two
