Abstract: A polarimeter comprises a polariser and an analyser between which is disposed a sample whose optical activity is to be determined. Light passes through the polariser, sample and analyser to a light detector responsive to the intensity of the light transmitted by the analyser. The polariser has a fixed orientation and the analyser is rotated by a motor which is driven in synchronism with digital clock pulses. The phase of the output signal from the light detector is compared with the phase of a digital signal derived from the clock pulses to determine the optical activity of the sample.