An instrument has been developed for sorting biological cells. The cells are rendered
differentially fluorescent and incorporated into a small liquid stream illuminated by a laser
beam. The cells pass sequentially through the beam, and fluorescent light from the cells
gives rise to electrical signals. The stream is broken into a series of uniform size drops
downstream of the laser. The cell signals are used to give appropriate electrostatic charges
to drops containing the cells. The drops then pass between two charged plates and are …