A magnification method of measuring focal length

The method described depends mainly on the determination of the distance between the two positions which the lens occupies corresponding to the reciprocal magnifications - m and - I/m. Details of the method as applied to the measurement of the focal length of a small achromatic lens are given. The necessary precautions to be taken in making the measurements and the corrections to be applied when the curvature of field is taken into account are discussed.