and other Spectra with the Spectrum of the Aurora. 259 



(see Plate V. spectrum 6). The principal lines in this spec- 

 trum were the line a (by far the brightest) corresponding to y 

 in the violet pole ; next was /3, a line in the yellow not appear- 

 ing in the tube-spectrum, and then other lines of less intensity. 

 In the " aurora"- and " phosphorescent " tubes was found, as 

 before mentioned, a line prominent for its brightness, and, in- 

 deed, in the " aurora "-tube the only one which survived when 

 it was moved away from the slit. This line also appears in the 

 spark-spectrum, but there only of an average brightness. I ex- 

 amined it carefully for position in the respective tubes, and on 

 comparing them found it coincident with the ridge or centre of 

 the wedge-like bright green broad band which is so conspicuous 

 in the air-tube spectrum. 



I think this edge-like centre has actually a line coincident 

 with the line I refer to ; but if so, its intensity little exceeds that 

 of the band itself. 



To complete the set of air-experiments, I examined the same 

 spark taken from the surface of a small meniscus of water placed 

 upon the lower platinum wire. In this case the air-spectrum 

 was plainly, but not brightly, seen at the violet end of the 

 spectrum ; the red, yellow, green and blue being filled with a 

 continuous spectrum through which some of the air-lines faintly 

 showed (see Plate V. spectrum 7). I reserve the remarks on the 

 position of the air-lines as compared with those of the aurora till 

 later. 



Phosphoretted-Hydrogen Flame. 



This was obtained from an ordinary hydrogen-bottle fitted 

 with glass tubing, two or three minute pieces of phosphorus 

 being placed with the zinc. The flame was of a bright yellow 

 colour with a vivid cone of green light in its centre. 



The spectrum was found to consist mainly of three bright 

 bands in the yellow, green, and green-blue respectively (see 

 Plate VI. spectrum 1). 



The centre band was very striking in its emerald-green colour, 

 while all the bands were remarkable as being very broad in pro- 

 portion to the slit (which, however, was not fine). The yellow 

 band had also a rich glow of colour. My spectrum was mapped 

 out at ordinary temperature, and I found the bands sufficiently 

 bright; but Mons. Lecoq de Boisbaudran, in his 'Spectres 

 lumineux' (Texte, p. 188), has described how the brilliancy of 

 these bands is increased when the flame is artificially cooled 

 (refroidie) . He also makes the important remark that the rela- 

 tive intensities of the bands are in such case altered, adding : — 

 " La plus importante de ces modifications consiste en un ren- 



