BY suspending a 6 mm. diameter circular half-wave plate horizontally from a quartz fibre with a torsional constant of 2 × 108 dyne cm. per radian, and allowing circularly polarised light to traverse it, thus reversing the direction of rotation of the electromagnetic light-vectors, we were able to observe torques of amounts plus and minus 2 × 1011 dyne cm., according to the direction of rotation of the incident light. The circular polarisation of the incident light was produced by passing plane polarised light through a quarter-wave plate. By rotating this quarter-wave plate through 360°, the light could be polarised twice in a clockwise direction and twice anti-clockwise.