An optically recording flow meter has been devised which accurately registers the phasic blood flow in the various vessels of the heart.The rate of flow is measured by directing the blood through a thin orifice plate (or Pitot tube arrangement). The momentary acceleration of the blood so produced causes a lowering of the lateral pressure (for a short distance downstream) which is proportional to the square of the acceleration. The moment to moment differences between the lateral pressures above and below the orifice, recorded by a differential manometer, give the phasic changes in the rate of blood flow.The upstream lateral pressure is led to a manometer tip with a 4 mm opening covered with a special rubber membrane 0.003 to 0.006 inch thick stretched 3 to 5 times. A waterproofed piano mirror (chip from a Bureau of Standards certified counting chamber cover slip) is mounted on the membrane by a peg or splint with special cement (to prevent hysteresis). The downstream lateral pressure is led directly to a ...