P lates 2-4.] W hen the oscillating electric spark is examined in a rapidly rotating mirror, the successive oscillations render themselves evident in the image as a series of luminous curved streamers which emanate from the poles and extend towards the centre of the spark gap. These streamers were first observed by F eddersen# in 1862, but the work of Schuster and HEMSALECHf in 1900 may be said to have opened up a new era in the subject. These workers threw the image of the spark on the slit of

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... a spectroscope, and photographed the resulting spectrum on a film which was maintained in rapid rotation in a direction at right angles to th a t of the incident light. In their photographs they found th a t the air lines extended straight across from pole to pole, but th a t the metal lines were represented by curved bands drawn out in the centre of the spark gap. There is a close relation between these bands and the streamers seen in the unanalysed inductive spark. Schuster and H emsalech carried out their experiments with the smallest possible inductance in series with the spark, and thus made the period of the oscillations so small th a t the drawing out on the film was insufficient to separate the individual oscillations from each other. Thus their curved lines represent a composite structure, consisting of all the streamers due to the successive oscillations superposed on each other. It follows from their results th at the light of the streamers in the spark is entirely produced by the glowing of the metallic vapour of the electrodes, and that, while the luminosity of the air is practically instantaneous in its occurrence, th at due to the metal vapour occurs in the centre of the spark gap an appreciable time later than near the poles. The actual process which goes on in the spark and gives rise to this delay in the arrival of the metallic vapour at the centre of the gap is not yet thoroughly understood. Schuster and H emsalech make the natural supposition th at it is due to the fact th at the metal of the electrode is vaporised and rendered incandescent by the heat of the spark, and th at the vapour takes an appreciable time to diffuse from (444.)