Measurement of Focal Lengths of Lenses

The apparatus generally employed to determine the focal length of a lens is that known as the optical bench.

It consists simply of a horizontal scale of considerable length, mounted on a substantial wooden beam, along which upright pieces can slide, and to these are severally attached the lens, the luminous object, and a screen on which the image formed by the lens is received. These sliding-pieces carry verniers, by which their position with reference to the scale can be determined. The position of each face of the lens relatively to the zero of the vernier is known or can be found as described on p. 261.