1882.] on Comets. 11 



much importance to the fact that the bright groups in the visible 

 spectrum of comet h agreed with those of the so-called " flame 

 spectrum," for the reason that the same sj)ectrum may be obtained 

 from the induction spark, when suitable arrangements are used to 

 make the discharge one of comparatively low temperature.* 



As we are now fairly on the wide ocean of speculation, I need not 

 say that the j)recise modes of application of the principle of elec- 

 tricity which have been suggested are many. Broadly, they group 

 themselves about the common idea that great electrical disturbances 

 are set up by the sun's action in connection with the vaporization 

 of some of the matter of the nucleus, and that the tail is matter carried 

 away, possibly in connection with electric discharges, in consequence 

 of the repulsive influence of the sun, which is supposed to be in a state 

 of constant high electrical potential of the same name. Further, it 

 is supposed that the luminous jets and streams and caj)S and envelopes 

 belong to the same order of phenomena as the aurora, the electrical 

 brush, and the stratified discharges of exhausted tubes. Views 

 resting more or less on this basis have been suggested by several 

 physicists, and, in particular, have been elaborated at great length 

 by ZoUner, who endeavours to show that on certain assumed data, 

 which appear to him to be highly probable, the known laws of 

 electricity are fully adequate to the explanation of the phenomena 

 of comets.f 



All the theories we have considered assume that the bright lines 

 seen in the spectra of comets indicate heated luminous gas. An 

 alternative hypothesis has been suggested by Professor Wright, J and 

 especially by Mr. Johnstone Stoney,§ who considers that the comjDound 

 of carbon vaj)our is opaque in reference to the particular rays which 

 appear as bright lines, and they appear as bright lines in consequence 

 of sending back to us the sun's rays falling upon the vapour. 

 Further, he considers the phenomenon to be of the order of phos- 

 phorescent bodies, and he states that the conditions existing in the 

 cometary gas are such as will eminently promote j)hosphorescence, and 

 therefore visibility, in presence of a luminary. || 



Here I must stop. May I venture to hoj)e that the experience of 

 the past hour has not been such as to confirm in your minds the old 

 view to which I referred at the beginning of the lecture, that the 

 influence of comets is always a malign and woeful one. [W. H.] 



* See Professor Piazzi Smyth, ' Nature,' vol. xxiv. p. 430. 



t • Astr. Nachr.' Nos. 2057-2060, 2082-2086, and ' Ueber die Natur der 

 Cometeu,' Leipzig, 1872. 



X ' American Journ. S. and A.' vol. x. July 1875. 



§ British Association Report, 1879, p. 251. 



II Respighi (' Comptes Rendus,' 5 Sept. 1882) has sought indeed to explain the 

 occurrence of bright bands by supposing them to be simply tiie remaining portions 

 of the continuous spectrum of reflected sunlight after absorption through the 

 enormous depth of the comet's atmosphere. This view appears to me for many 

 reasons improbable, especially if we take into account the extreme relative 

 brilliancy of the most refrangible group in the photographic spectrum of comets. 



