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.
Normally, the thread of the screw is half a millimeter and the disk is divided into 500 parts; in our exemplar the thread is 0.3774 mm and the disk is divided into 100 parts.
The thin edge of the disk acts as the index for the graduated scale adjacent to the disk on which the number of turns of the screw are read, while the angle of the scale adjacent to the disk, it too made thin, acts as the index for the graduation of the disk and allows evaluation of fractions of revolutions.
It is possible to produce micrometric screws so precise and without play as to allow appreciation on the disk of a ten-thousandth of a millimeter, but it is far more difficult to establish with the same precision the instant of contact of the point with the supporting plane.
The model presented here, when not in use, is placed on a wooden disk with three small indentures for the points supporting the instrument and is protected by a glass jar open at the top; the jar is covered by the plate of glass that acts as the supporting plane during measuring operations.
Battelli - Cardani (1922), Vol. I, p. 232
Daguin (1878), T. I, p. 28
Drigo - Alocco (1945), p. 231
Perucca (1949), Vol. I, p. 45
Violle (1883), T. I, p. 328