An outline of the method in which the projectile interrupts successively beams of light shining on photoelectric cells placed along the trajectory is giver. The velocity is measured by means of records taken of the times between interruptions of the beams of light. An area about 24 by 30 in. is made sensitive to the projectile by reflecting a beam of light across this area several times, then directing it on the photoelectric cell. The details of the apparatus are given.The recording apparatus consists of an amplifier which impresses the current pulse from the photoelectric cell on the grid of a thyratron when the Projectile interrupts the light beam. This thyratron, on discharging a condenser, am a short flash of fight which is recorded on a rotating drum camera. A similar thyratron controlled by a tuning fork puts a simultaneous time record on the film. The camera is so designed that the film can be taken out and developed in daylight.The results of firing tests in Petawawa indicate that the apparatus can be used successfully in daylight with field guns. The present equipment measures velocities with a probable error of less than 2 ft. per sec. for muzzle velocities of the order of 1600 ft. per sec. Improvements in design are discussed.