Abstract: According to the invention, the instrument for measuring the sizes and quantity of particles in a fluid medium comprises a light-sensitive device which converts light scattered by particles in a flow of a fluid medium which crosses a beam emitted by a light source, as well as some part of the light of that beam, arriving from an element whose purpose is to simulate a calibrated particle, to electric pulses. The light-sensitive device is connected via a divider, whose purpose is to distribute pulses according to the sizes of particles, to signal inputs of respective threshold devices whose outputs are connected to inputs of counters with indicators. The element which simulates a calibrated particle is adapted to periodically change, in the course of measurement, the brightness of light received by the light-sensitive device.