(2) The direction of the current in the tube was then changed; and, without the magnet, the ring electrode was surrounded by a diffused violet glow; while the straight wire gave forth a faint salmon-coloured stream of light, spreading up to the ring. [Sidenote: Magnet effects described. On negative pole. Rings from positive pole described. Effects on rings of making and breaking contact with magnet. Shape of rings described.] On excitation by the magnet (the positive pole being now placed between the armatures), the violet glow of the negative pole contracted into a compact mass round the ring electrode. At the same time from the positive pole sprang a set of bright saddle-shaped rings, which increased in size as they advanced; and spreading upwards with a rapid but smooth motion towards the negative pole, closely approached to, but never actually came in contact with, the violet glow. The positive end of the tube was otherwise but slightly lighted, and the sudden appearance of this brilliant stream of rings of light was very striking. A single bright ray was also seen running from the positive wire, in a somewhat transverse course, along one side of the tube. When wire-contact with the magnet ceased, so that it was not excited, the rings ran back in succession to the positive pole and disappeared, and by making and breaking contact they were caused to advance and retire at will. They were accompanied by a waving or vibratory motion, and were evidently of the same character as the smaller striæ or bands mentioned as seen when the ring formed the positive pole. The general appearance was that of a hollow cone of light (the base towards the negative pole), composed of brilliant rings with dark spaces between, which appeared and expanded under the magnetic influence, and contracted and disappeared on its removal. The rings did not appear to be flat disks, but were somewhat curved or saddle-shaped. They reminded one much of the diatom _Campylodiscus spiralis_; that is to say, they were apparently flat if looked at from above, but like a figure of 8 when viewed sideways, the peak of the saddle forming a kind of brilliant point or apex. All this is difficult to describe; but an illustration from a sketch made of the tube is given on Plate XVII. fig. 2. [Sidenote: Negative pole placed vertically on the magnet.] (3) The negative pole (straight electrode) was then placed vertically on one of the poles of the electro-magnet. On excitation, the violet glow was contracted into a small upright brush or column of bright light, with a slight inclination to curvature. [Sidenote: Tube laid horizontally across poles of magnet.] (4) The same Plücker tube was laid horizontally across the poles of the electro-magnet (without armatures), the respective electrodes being above each pole. [Sidenote: Effects produced.]