782 THE AURORA BOREALIS. 



I do not know if these several phases of the positive part of 

 the discharge have all been examined spectroscopically. They 

 pass into each other according to the shape and size of the tube 

 or flask, as well as the air-pressure ; and it is difticult to say how 

 much of each is concerned in those observations which have been 

 made of air-spark spectra in comparison with the aurora. No 

 one, so far as I know, has compared with it the negative-glow 

 spectrum so fully as Angstrom has now done ; and it seems very 

 probable that its peculiar fitness for the comparison has been 

 overlooked — the feat of filling a bottle with the negative glow 

 discharge being certainly a novelty ; if it is really true that he 

 succeeded in obliterating the positive brush entirely in its favour. 



The next remarkable novelty in the paper is the way in which 

 he proposes to account for the " citron " line of auroras ; for 

 there is evidently nothing of the kind in the negative glow, how- 

 ever well that answers to all the secondary facts of faint blue, red, 

 and greenish lines. If oxygen and its compounds are (as has, I 

 believe, been lately shown) strongly fluorescent^ Tait and Dewar 

 have also proved, as shown by some of their experiments this 

 year, that they also possess powers of 'phosphorescence — Geissler 

 tubes shining for some time after the spark has passed through 

 them, from the production of ozone during the discharge. When 

 one of the globes of a phosphorescent " garland " tube was heated 

 over a Bunsen flame, that globe which was heated did not shine 

 after the spark had passed, apparently because, as we know, a very 

 little heat is sufficient to destroy ozone. Whatever the way may 

 be in which the ozone or otherwise electrified gas remains self- 

 luminous after the discharge, it seems very reasonable to suppose 

 some action of the same kind (perhaps, as Angstrom says, simply 

 fluorescence) as common in all auroras, and that this produces the 

 well-known auroral line. 



Pocket spectroscopes can, of course, do nothing further to fix 

 the position of the citron line ; nor can they alone fix very 

 exactly the places of any of the fainter ones. But as every 

 Aurora shows this strong monochromatic light, it might be used 

 to bring out a row of punctures transverse to the slit, as a divided 

 scale in the field of view whereby to map the fainter lines, or at 

 any rate to recognise those which appear most frequently. For 

 this purpose they should be made large, and the slit should be a 

 wide one. For ordinary miniature spectroscopes, two holes on the 

 red and five on the blue side of the slit, -^-^ or ^L of an inch apart, 

 would suffice for recognitions and even for very useful measure- 

 ments. The jaws of the slit can be cut with a fine saw across the 

 middle about -^^ or \ of an inch deep each way ; and a piece of 

 copper foil, provided with the row of holes and a sufficiently wide 

 slit across it, can be fastened to one of them inside, opposite to the 

 crosscut and adjoining the edges of the spectroscope jaws. 



Some other means maybe found of piercing the jaws of a pocket 

 spectroscope at regular intervals ; but, as a simple plan, I have 

 found this very efficient, in finer divisions, for laboratory use. 

 The holes are pierced at yj^ of an inch apart ; and thirty of them 



