In previous papers I have described experiments on the rocking of bodies. The rocking bodies have bearing surfaces which are plane, circular, or elliptical and they may rock on circular, elliptical, or plane surfaces.
Lenses also furnish interesting examples of rocking bodies. Their surfaces may be spherical, cylindrical, or plane and these have the advantage that they are smooth and of good geometrical form. Their curvatures may be measured accurately with calipers and spherometer. To lenses may be added prisms and mirrors. Odd lenses, prisms, and mirrors usually abound in a well-stocked laboratory so that examples are not far to seek. The larger the specimens the more easily do they lend themselves to experimentation. The calculations of the period of rocking furnish, as in earlier papers, interesting examples of the applications of mechanics to vibrational problems, and the calculation of positions of centers of gravity and values of moments of inertia provide additional exercises. The experiments are eminently suitable for senior classes in the physical laboratory, giving interest to the student by their novelty and little trouble to the demonstrators in the matter of putting out the apparatus for the use of the class. Their number illustrates the variations possible with very little apparatus.