A rotation viscometer, directly measuring the ratio of the shearing stress to the rate of shear, is described in detail. The reading, for Newtonian liquids, is independent of the speed of rotation and, for non‐Newtonian liquids, gives a direct indication of the dependence of the apparent viscosity on the rate of shear. The principle of the measurement is as follows: A slit, the width of which is proportional to the shearing stress, rotates together with the rotating cylinder of the viscometer; a narrow beam of light passes through this slit and falls on a photoelectric cell; the time of illumination of the photoelectric cell is proportional to the viscosity. The instrument may be used over a wide range of viscosities and rates of shear.