The spherometer is an instrument for the measurement of small thicknesses which, owing to its special structure, can also be used to measure the radius of curvature of a spherical cap. Its inventors, the opticians Cachoix and de Laroue, used it in measuring the radius of the curvature of lenses, thus its name.
It is composed of a steel micrometric screw with a very fine point, the nut screw of which is placed at the centre of a brass tripod the sharply pointed feet of which touch the supporting plane at the vertices of an equilateral triangle; the micrometric screw is perfectly perpendicular to the supporting plane and the point is equidistant from the points of the feet of the tripod.
With the spherometer what is measured directly is the distance between the point of the micrometric screw when it is in a certain position and the supporting plane.
One of the tripod feet is surmounted by a small graduated scale on which turns the thin edge of a horizontal brass disk, which has its centre at the axis of the screw and is joined to it. ...continues