A brief discussion is given of the theoretical prediction that the mechanical forces exerted by light on an opaque body are proportional, other things being equal, to the refractive index of the medium in which the body is immersed. An experiment to confirm this prediction is described. Light falls asymmetrically on a metal vane mounted on a torsional suspension. The normal pressure of the radiation on the vane gives rise to a mechanical couple, resulting in a small deflexion which is observed by means of an optical lever amplifier. The suspended system is mounted in a container which can be filled with various liquids, and the pressure on the vane when it is immersed in a liquid is compared with the pressure on the vane in air. The intensity of radiation entering the container is the same in each instance; after the small disturbing effect of the liquid on the optics of the system has been allowed for, the ratio of the two pressures is found to be equal to the refractive index of the liquid, within the limits imposed by experimental error of $\pm $1$\cdot $2% r.m.s. This result is obtained for six different liquids.