1889.] of the various Species of Heavenly Bodies. 225 



than accidental, unless the absence of the two brightest bands can be 

 explained. As this cannot be done, the suggestion may be dis- 

 regarded. 



The information given about the green line seen in the phosphor- 

 escent tube by Capron is insufficient for any conclusions to be 

 founded on it. 



Fig. 16 is a map showing that the aurora spectrum is not that of 

 the negative or positive pole of nitrogen, or any spectrum of oxygen, 

 although there are some apparent coincidences. The intensities of the 

 lines and bands in the spectra are indicated by lengths, the longest 

 being the brightest. The map shows that lines Or flutings as bright 

 as or brighter than those which have been supposed to coincide with 

 lines in the aurora are absent from the aurora spectrum. The probable 

 meteoritic origins, which I shall have to refer to in detail later on, are 

 shown at the bottom of the map. 



Groneman's reference to the Meteoric Dust Theory. 



So far we have had chiefly to deal with theories in which the 

 aurora spectrum is regarded as being inseparable from that of 

 atmospheric air, but we have next to consider one which, if true, 

 would give a totally different origin. 



In 1874, Groneman ('Astr. Nachr.,' No. 2010) resuscitated the 

 theory of Olmsted that the aurora has its origin in the fall of incan- 

 descent meteoric dust.* The iron particles are regarded as being 

 competent to produce the magnetic phenomena which accompany 

 auroras, and as being consistent with their geographical distribution. 

 This theory, however, was not received very favourably, because it 

 left the spectroscopic phenomena as far from a solution as ever. Thus, 

 Capron remarks (' Auroras,' p. 170) that "if auroras were composed 

 of incandescent glowing meteors, it would be reasonable to expect to 

 find in the spectrum the lines of iron, a metal constituting so 

 prominently the composition of meteorites. No connexion between 

 the iron and the aurora spectrum is, however, proved ; though it may 

 be suspected. The iron spectrum contains so many lines that some 

 may, as a mere accidental circumstance, closely agree with the aurora 

 lines." Yogel also considers that we are not entitled to regard the 

 close coincidences of the aurora lines with some of the iron lines as 

 complete evidence of iron vapour, until we have succeeded in showing 

 by experiments that the relative intensities of the iron lines are 

 subject to great changes ; and in this way to account for the appear- 

 ance of faint lines in the aurora spectrum, or, on the other hand, to 

 account for the absence of the strongest lines. I shall show subse- 



* This theory was subsequently discussed in an appendix to the ' Memorie della 

 Societa degli Spettroscopisti Italiani,' 1878. 



