In a recent note to the Royal Society of Edinburgh, Dr Dawson Turner remarks on a peculiarity observed by him in the behaviour of an inductioncoil spark-gap when a piece of mica, glass, sulphur, ebonite, etc., is placed between the terminals, near or against the + one. The surprising variety of the phenomena exhibited on the interposition of a solid dielectric plate between the two poles of such a coil (or between the poles of an influence machine) may make, in view of Dr Turner's note, the following observations of some interest. The observations in question were made during the course of an inquiry into the causes of the Lullin effect (according to which a thin dielectric plate interposed between two sparking terminals not opposite to one another is, as a general rule, perforated at the negative pole), and the apparatus employed consisted essentially of the details shown in fig. i., viz. of a motor-driven influence machine whose terminals were connected through a battery of Leyden jars in cascade to a specially designed spark-gap which allowed of any three-dimensional motion of the electrodes being accurately measured, and of any desired type or form of electrode (spherical, conical, disc type, etc.) being inserted at will.