ON THE ORIGIN OF THE PLANETARY SYSTEM. 159 



a spot according to Padre Secchi, as seen under power- 

 ful magnification. Their diameter is often more than 

 many tens of thousands of miles, so that two or three 

 earths could lie in one of them. These spots may 

 stand for weeks or months, slowly changing, before 



FIG. 10. 



they are again resolved, and meanwhile several rota- 

 tions of the sun may take place. Sometimes, however, 

 there are very rapid changes in them. That the core 

 is deeper than the edge of the surrounding penumbra 

 follows from their respective displacements as they 

 come neat the edge, and are therefore seen in a very 



