Abstract: An instrument functioning as either a turbidimeter or as a depth meter for measuring the distance from the instrument to the level of a turbid fluid. A light source provided with an optical system emits a beam of light to the fluid, and the volume or "beam of vision" viewed by a photo-cell likewise provided with an optical system intersects said beam of light. The two beams include a relatively small angle so that a tapered or conical volume of intersection in the fluid is obtained. Thus, the quantity of light scattered or reflected within this volume and reaching the photo-cell is dependent on the turbibity and the position of this level. The light emitted by the light source is modulated at a relatively high frequency. The output of the photo-cell passes through a frequency filter and is, then, indicated or recorded as a measure of turbidity or of depth of said layer.