P. A. Secchi on KirchhofFs Theory of Solar Spots. 44S 



Again, to treat Wilson's theory as absurd shows that the same 

 is confounded with that of Herschel, from which, in reality, it 

 differs essentially. Wilson merely pronounced the spots to be 

 cavities ; and subsequent observations have proved this to be a 

 fact. But no one said that these cavities were absolutely empty ; 

 on the contrary, the cirri seen across them show that the cavities 

 are filled with less incandescent gaseous matter, which occasionally 

 is visibly agitated, and thrown into vortices and currents. This 

 being the case, what else can these cavities be, if not spaces full of 

 less luminous and consequently less incandescent matter ? Prof. 

 Kirchhoff considers them to be clouds or masses at a lower tem- 

 perature. When well considered, there is in reality not much dif- 

 ference between the two hypotheses, provided the terms are well 

 defined. This difference is still further diminished when we find 

 the origin of these clouds attributed to vortices and cataclysms, 

 to which causes we, too, have frequently attributed the origin of 

 spots. 



The only controverted point which remains to be decided is 

 whether the dark spot termed the nucleus is a portion of the 

 general mass under the photosphere, or whether it is produced 

 by the opacity of a cloud or more refrigerated mass, which inter- 

 cepts from us the rays emitted by the subjacent luminous portion. 



This question cannot be decided a priori ; it will require an 

 examination of the forms and phases of the spots themselves. 

 Now their forms do not at all agree with those of clouds — so 

 far, at least, as we can judge from the phenomena of our own 

 atmosphere, and from the properties which an incandescent 

 atmosphere, like that of the sun, may be reasonably supposed 

 to possess. 



In fact at the commencement the spots resemble black pores, 

 which certainly suggest, to some extent, the idea of clouds. 

 Every analogy, however, soon disappears; for as soon as the 

 pore, by dilatation, assumes the appearance of a spot, its edges 

 are seen to be serrated, and the penumbra is formed entirely of 

 very fine rays converging towards the centre of the figure. As 

 often remarked, the nucleus does not always present exactly the 

 same contour as the penumbra, but to a salient angle of the lu- 

 minous matter against the nucleus corresponds a reentrant one 

 in the penumbra, exactly as if a descent of matter into the nu- 

 cleus had taken place from its sides, and left an escarpment 

 (talus) the more reentrant the more copious had been the de- 

 tachment of matter. These are the phases through which all 

 spots pass until they reach the first stage, which appears to be 

 that of formation and complete development ; to this succeeds 

 the phase of dissolution. 



Accordingly it is manifest that this first phase presents no- 



