(5) The apparatus was now changed for one of the following arrangement:—1, a rotating Nicol prism next the eye; 2, the glass bar; 3, a biquartz with the halves horizontal; 4, another Nicol prism. The neutral-passage tint of the biquartz was found to be rather green (from mixture with the yellow of the glass). [Sidenote: Change in colour of the halves.] (i.) Placed _along_ the poles of the magnet and the magnet excited, a change of tint was seen in both halves of the biquartz, the slightly purple-reddish tint of the upper half passing into a full purple. Effect not so marked as with the double-image prism. (ii.) Placed _across_ flat ends of the armatures (as in experiment No. 4) no effect was seen. CHAPTER XVI. EFFECT OF MAGNET ON WIDE AIR (AURORA) TUBE. [Sidenote: Wide air-tube described.] A large, wide air-tube was tried; it was 14½ inches long by 1 inch in diameter, of the same bore throughout, and with straight platinum electrodes. [Sidenote: Magnet effect when tube placed vertically between conical armatures.] (1) To excite it the larger coil was used. The tube was filled with bright, steady, rosy light, and beautiful stratification, which, as it flickered, seemed to incline to a continuous spiral (Plate X. fig. 8). This stratification was very close and fine, and extended nearly throughout the tube. On excitation of the magnet (the tube having been placed _vertically_ between the conical armatures), the glow was condensed into a bright solid line or stream of light at the point which lay directly between the poles. This line or stream expanded into an elongated funnel-shape as it retreated from this centre towards the extremities of the tube, the stratification showing itself more distinctly as the glow of light became less dense (Plate XVIII. fig. 3). The stream of light was driven away at right angles to the poles, and changed from side to side of the tube with the direction of the current. [With the small coil this tube showed only a flickering stream of light, with very slight indications of stratification.] [Sidenote: Effect when tube placed horizontally between the armatures.] (2) The tube was placed _horizontally_ between the conical ends of the armatures. The condensed stratified stream of light flew upwards and downwards (according to direction of current) instead of to the respective sides of the tube. [Sidenote: Tube placed along the poles of the magnet.] (3) The tube was placed along the poles of the magnet. In the interval between these the stream was driven upwards, but at either end sideways, right or left according to whether the pole was N. or S. (Plate XVIII. fig. 4). The result gave a complete spiral of stratified condensed light within the tube. _Note on Stratification._ [Sidenote: Stratification in small tubes arranged in series.]