An apparatus for measuring optical rotation produced by optically active substances is described in which light, after passing through a continuously rotating Polaroid disk, carrying a degree scale, then goes through a stationary Polaroid disk and is incident as sinusoidally modulated light upon a phototube, causing it to produce a signal (of audio‐frequency twice that of the frequency of rotation) which is then passed through an audio‐amplifier and fed to a stroboscope, directed on the rotating disk, which thereby flashes in synchronism with the signal, and hence also with the rotation of the disk. By adjusting the angular position of the stationary disk, the zero mark on the rotating disk is made to appear to coincide with a stationary fiducial mark. When an optically active sample is interposed between the polaroid disks, there is a phase shift in the modulation of the light which causes a reading, as observed in stroboscopic light, on the rotating disk equal to the angular rotation produced by the sample.