1. Previous Experiments .—In the year 1899 Professor Schuster and Dr. Hemsalech published the results of their experiments on the Constitution of the Electric Spark. On a slit parallel to the spark they formed an image of a spark produced between metal electrodes by a Wimshurst machine and photographed the light passing through the slit, both directly and after analysis by a prism, on a moving film in the focal plane of the camera lens, the direction of motion of the film being at right angles to the direction of the slit. A summary of their results is given below:— (1.) In the photographs on the moving film the lines in the spectrum which are due to the metallic vapour were inclined, whilst the air lines were straight.