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The spark is produced by a small induction-coil, known as du Bois-Reymond's chariot apparatus; it strikes between two blunt points of platinum-iridium, carefully machined and polished. These are fixed to the two jaws of a pair of wooden pliers, which tend to close by elasticity, and are kept apart by a micrometer screw. At a distance of about 2 cms. from the spark, and facing the plate, a plate of ground glass is fixed. As I have previously mentioned, the light of the spark produces on this glass an extensive luminous patch, much easier to observe than the naked spark, and giving on the photographic plate impressions of much more regular form. The regulating of the spark is the delicate part of the experiment. The induced current must first be adjusted, by modifying the primary current on the one hand, and the position of the secondary coil on the other, till the spark becomes very small. The points are washed in alcohol, then a slip of dry paper is drawn between them, for the purpose of drying and repolishing their surface; then the micrometer screw is turned so as to make the spark as short as possible, yet without