Abstract: Particle distribution in a fluid flowing through a microscopic tube is achieved by flowing the fluid vertically-downward through a transparent capillary tube and passing light through the flowing fluid. A linear array of photodiodes responds to light passing through the fluid by registering a series of signals representing the linear projection of particles passing through the plane defined by the light source and the photodiode array. The fluid is supplied from a reservoir which can be selectively pressurized by a gas to control the egress flow rate of fluid. A syringe serves as the egress path from the reservoir and a rotatable stirring rod is disposed in the syringe to stir the egressing fluid. The syringe feeds a disposable assembly which includes a hypodermic needle feeding the capillary tube which is mounted between a microscope slide and cover in the path of the sensing light.