1889.] of the various Species of Heavenly Bodies. 241 



There are only six lines for which no origins can at present be 

 suggested. The discrepancies between the readings of the same lines 

 at different times are so great that a few outstanding lines are only to 

 be expected. 



It now remains for future observers to determine by direct com- 

 parisons whether the coincidences suggested are real, or merely 

 accidental approximations. 



VII. The Norwegian Observations. 



The Report of the Norwegian Polar Station at Bossekop in Alten, 

 in connexion with the International Polar Investigation (1882-83), 

 gives the results of a few interesting observations of the aurora 

 spectrum. Herr Krafft states that in general only the characteristic 

 aurora line (558) is seen, even in strong aurorse. The red line 

 occasionally appears very conspicuously, but only in flashes. 



The wave-lengths obtained for the aurora line were 5595, 5586, 

 and 5587. Unlike most observations, these place the aurora line on 

 the less refrangible side of the manganese fluting. Hence, we have 

 an additional reason for neglecting the difference between the wave- 

 length of the brightest edge of the manganese fluting, and the 

 commonly accepted wave-length of the aurora line, as given by 

 Angstrom. 



On account of the rapid flashing-up and disappearance of the red 

 line only one measurement could be made, and the wave-length 

 obtained was 6205. If this reading be reduced in the same propor- 

 tion as those of the green line, a wave-length is obtained which 

 agrees almost perfectly with that of the brightest edge of the iron 

 fluting.* 



These observations are the latest which have been published, and 

 were obviously made with a full knowledge of all previous work, so 

 that their importance must be strongly insisted upon. 



It is fair to assume that the red line is due to iron, because we know 

 that the effect of a slight increase in the intensity of the discharge 

 which produces an aurora in which only the manganese fluting is 

 visible would be to bring out the iron vapour. Hence in an aurora 

 in which the green line is constant, and the red line is only inter- 

 mittently visible, there must be a discharge in which there are sudden 

 fluctuations in intensity, and a simple cause of the reddening or the 

 aurora is now before us. 



VIII. The Spectrum of Lightning. 



If the origin of the auroral spectrum is really that which I have 

 assigned to it, in lightning in which the electric action is feeble we 



* These observations were not available to me before the preceding maps were 

 made, so that the iron fluting has been omitted from them. 



