A mechanical stroboscope consisting of a disk whirling at 10 revolutions per second with a radial slot cut into it rotates in front of a cameras lens making a record of this scene of a young lady lowering her arms. With each successive passage of the slot on the rotating disk her arms are in a lower position. Since the shutter is held open during this time the various positions of her arm are recorded sequentially lower in position with the passage of time.