The measurement of distances of image and object is repeated,
using both lenses and with the lens and the screen in different
positions.
2. If, at a fixed distance d between object and image (case I),
we alter the position of the lens so that the image and object
distances are transposed (case II), we still obtain a clear
image of the object. In case I the image is magnified, in case
II it is reduced (Fig. 2).
Fig. 2: Determination of focal length after Bessel