When data with and without an optically active sample are acquired simultaneously while one manually rotates the analyser, the graph of the first signal versus the second one is an ellipse whose shape shows the phase shift between the two signals; this shift is twice the optical rotation. There is no need to measure the rotation of the analyser or to uniformly rotate it. For small optical rotations, much better resolution is obtainable than in simple measurements based on minimizing the transmittance of light passing through the polarizer and analyser. (Contains 5 figures.)