258 Mr. J. R. Capron on the Comparison of some Tube 



is the brightest of the violet-pole group, and which represents 

 a medium-intensity band in the capillary spectrum. After this 

 is a faint band near a, representing two rather bright ones in the 

 capillary spectrum, this last being succeeded by other bands in 

 the violet, a, @, and y in the violet-pole were examined carefully 

 for relative brightness, and are, I believe, correctly marked. 



The red- or 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, it is also a fair representation of a dif- 

 fused air-spectrum (see Plate V. 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 retaining their promi- 

 nence; but after a I judged e next in brightness. On examining 

 the violet pole at 12 inches from the slit, the whole spectrum was 

 faint, and the bands a and /3 were alone distinctly seen. 



Next to the Geissler air-tube I examined an " aurora "-tube, 

 about 15 inches long and 1 J inch across, with platinum terminals 

 and of the same diameter throughout. The discharge was of a 

 rosy red colour; and the long flickering stream from pole to pole 

 certainly much reminded me optically of an auroral streamer. 

 Spectroscopically examined, the discharge presented a faint 

 banded air-spectrum similar to that of the positive pole (see 

 Plate V. 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 spec- 

 trum, and in this respect distinguished it from the ordinary air- 

 spectrum. 



Following this last tube I examined one purchased as " phos- 

 phorescent." It was rather short (6^ 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 lines or 

 bands in the blue and violet portions of the spectrum (which I 

 have not yet worked out), 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. 



1 next took a ^-inch spark in air between platinum terminals 



