P. A. Secchi on KirchhofPs Theory of Solar Spots. 445 



thermoscope. At the same time it is manifest that the source 

 of these gaps is rather an eruption proceeding from the interior 

 of the nucleus, than a simple refrigeration produced in the pho- 

 tosphere by causes analogous to our own meteorological ones. 

 In fact it is difficult to imagine the existence of the latter in 

 the sun. whilst internal eruptions cannot fail to exist under the 

 conditions to which that body is exposed. 



Again j just as Herschel, in order to explain the penumbra?, 

 was induced to assume two strata in the photosphere, so Prof. 

 Kirchhoff requires two strata of clouds, necessarily coexistent and 

 superposed one on the other. This hypothesis of two strata is 

 in truth a mere expedient for the explanation of the penumbras, 

 whose constitution we have already described, and to explain 

 which it is not necessary to introduce more than a simple pho- 

 tosphere, together with the accessories inseparable therefrom. 

 The cloud hypothesis has often been reproduced, but always by 

 those who have either observed the spots with imperfect instru- 

 ments, or who have not studied them very carefully. By those 

 who have had at their disposal better methods of observation, 

 the hypothesis has always been rejected. 



Neither is it at all the object, as we too frequently hear, of 

 the supporters of the hypothesis of a less luminous nucleus to 

 revive the old notion of the habitability of the sun ; for if the 

 Creator wished to make that planet habitable, He would not 

 necessarily have to people it with men of flesh and bone like 

 ourselves, who would be volatilized in a few instants ; nor for 

 this purpose is it necessary to suppose that the dark stratum 

 acts as a tent to protect those imaginary inhabitants from the 

 superior radiation. Such arguments have their appropriate place 

 in treatises such as those of Fontanelle and his imitators, where 

 they may serve to divert readers. 



We merely assert that, without contradicting physical laws, 

 first, the stratum of the photosphere may have a greater lumi- 

 nosity than the internal nucleus has ; secondly, it is not neces- 

 sary to suppose the so-called nucleus to be either solid or liquid ; 

 it may be gaseous and of greater density ; thirdly, notwithstand- 

 ing the proximity of the photospheric stratum, the nucleus may 

 have not only a different degree of luminosity, but also a dif- 

 ferent temperature; fourthly, the apparent form of the spots 

 absolutely excludes the hypothesis of a cloud structure, and we 

 see nothing in them adequately analogous to the formation of 

 terrestrial clouds, or to the phases through which they pass. 



Our object in the preceding remarks has been, not so much to 

 raise objections to the views of a distinguished physicist, as to 

 prevent a scientific retrogression ; for history shows that persons 

 of great authority in one branch of science — but of less eminence 



