ClipArt ETC
"An instrument for measuring the radii of spheres; a sphere-measurer. It is of especial service to opticians in determining focal lengths, etc. of lenses. The common form consists of a vertical screwc, with a large graduated head a, a, turning in a socket supported by three legs whose hard steel points are exactly equidistant. The fixed scale b at the side, together with the graduated screwhead, makes it possible to measure with great accuracy the distance between the extremity of the screw and the plane passing through the ends of the three supports, when, for example, all the points are in contact with the surface of the sphere." —Whitney, 1889

Spherometer

"An instrument for measuring the radii of spheres; a sphere-measurer. It is of especial service to opticians…