and other Spectra with the Spectrum of the Aurora. 257 



The Geissler tube was probably filled designedly with coal- 

 gas. In the case of the barometer-tube, the spectrum may be 

 assumed to be the result of some carbon impurity. 



No lines of mercury could be detected in either case. 



Air-Tubes. 



As the air-spark spectrum may be said in some form to repre- 

 sent lightning, so it would seem not unreasonable to expect a 

 rarefied-air spectrum to represent the aurora ; and indeed it has 

 long been the practice in lecture-rooms to exhibit with con- 

 siderable success exhausted air-tubes excited by electricity as 

 " aurora " tubes. The first tube I examined was an ordinary 

 Geissler tube charged with rarefied air. The bulbs, on passing 

 the discharge, were filled with the well-known rose-tinged light, 

 like to the aurora-streams, which in the capillary part was con- 

 densed into a brighter and whiter thread, while the platinum 

 wire of the negative pole was surrounded by its usual mauve or 

 violet glow. 



The spectrum, even with a weak current, was quite bright, 

 and consisted mainly of the nitrogen-lines and bands, with the 

 lines Ha, H/3, and Hy, and some of the intermediate lines of 

 the H spectrum. 



The double line a was undoubtedly the brightest in the spec- 

 trum when taken in the capillary part of the tube. After this 

 followed ft, and then 7 (H), 8, and e ; but I am uncertain as to 

 the relative brightness of the three, and mark their intensities 

 with hesitation. I tested them several times independently 

 with differing results, and I suspect them of variability with the 

 current. 



The rest of the lines are very much of the same intensity. (For 

 drawing of spectrum of air-tube in capillary part see Plate V. 

 spectrum 1.) 



I next turned my attention to the violet or negative-pole glow; 

 and here a remarkable change takes place in the spectrum, not 

 only in the position of the principal bands or lines, but in their 

 relative intensity (see Plate V. spectrum 2). 



The double line a in the capillary part is replaced in the 

 violet glow by a shaded band of second intensity ft, the sharp 

 edge of which is extended towards the red and forms (except 

 for some faint indications) the limit of the spectrum in that 

 direction. The somewhat faint line next a in the capillary tube 

 has its faint representative in the violet-pole ; but the next two 

 lines (capillary) are represented by the bright band 7 in the 

 violet-pole lying in a position between them. Next 7 in the 

 violet-pole come three faint lines representing ft, 7, and 8 in 

 the capillary spectrum ; and then comes the bright band a, which 



