Red (positive) pole: spectrum described.
The red [positive] pole was next examined, but presented no peculiar features. It appeared as a fainter representation of the capillary air-spectrum, with some few lines or bands absent, and (as will be seen after) was also a fair representation of a diffused air-spectrum (see Plate XV. spectrum 3).
Examined for comparative intensity, at 24 inches from the slit, the whole capillary air-spectrum showed faintly. The marked lines in the centre of the spectrum generally retained their prominence; but after α I judged ε next in brightness. On examining the violet pole at 12 inches from the slit, the whole spectrum was faint and the bands α and β were alone distinctly seen.
Aurora (air)-tube. (Plate XV. spectrum 4.)
Aurora-tube: discharge described. Spectrum described.
Next to the Geissler air-tube I examined an “aurora”-tube, about 15 inches long and 1¼ inch across, with platinum terminals, and of the same diameter throughout (Plate X. fig. 8). The discharge was of a rosy-red colour, and the long flickering stream from pole to pole certainly much reminded one optically of an auroral streamer. Spectroscopically examined, the discharge presented a faint banded air-spectrum similar to that of the positive pole (see Plate XV. spectrum 4); but the relative intensity of the lines was somewhat altered, while a very bright line in the green (seen also in the tube next described) was characteristic of the spectrum, and in this respect distinguished it from the ordinary air-spectrum.
Phosphorescent tube.
Phosphorescent tube described. Discharge described. Spectrum described.
Following this last tube I examined one purchased as “phosphorescent.” It was rather short (6½ inches), of equal calibre, and about the size of the bulb of a Geissler tube. It was filled with a white powder (probably one of the Becquerel compounds). On passing the current between the electrodes, a bright rose-coloured stream appeared; and wherever this was in contact with the powder, the tube glowed with a brilliant green light. On stopping the current, the tube still continued to shine, but with a fainter green glow, which gave only a continuous spectrum. When examined in full glow, the tube-spectrum was also in the main continuous and of a green tinge; but upon it were bright lines in the blue and violet portions of the spectrum, while in the red, yellow, and green a faint but distinct air-spectrum was seen; and with this was also found the same bright line in the green which distinguished the “aurora”-tube. [Five out of six of the lines in the blue and violet will be also found in Schuster’s oxygen-tube, violet pole (Plate XVIII. fig. 15). The air-spectrum probably arose from impurity.]
