A method of obtaining direct information as to the mode of development of the lightning discharge has been described by Boys. It consists in the use of a camera with a fixed plate and two lenses revolving in a circle at opposite ends of a diameter. The two pictures of the lighting flash thus obtained show distortions, due to the motions of the lenses, which are in opposite directions. From a comparison of the two pictures and a knowledge of the velocities of the lenses, it is possible to deduce the direction and speed of the developing discharge. Boys has published the results of an examination of one such photograph taken by him, in which the discharge began at the ground and almost immediately afterwards started in the length next the cloud. It then travelled from both these parts and finished in the middle portion about 140 microseconds later.