366 PHYSIOGRAPHY. [chap. 



called the peuumbra,^ while the darker shade represents the 

 umbra^ and, within the umbra itself, there may sometimes 

 be detected a yet darker part, which is called the micleiis. 

 There is reason to believe that these spots are nothing but 

 gigantic cavities, and that differences of shade correspond 

 to differences of depth, the nucleus thus representing the" 

 most profound part of the hollow. The intensely luminous 

 part of the sun, which is the seat of these spots, is called the 

 photosphere? It appears to consist of incandescent cloud- 

 like matter, which is subject to violent disturbances, whereby 

 depressions are produced, into which the solar atmosphere 

 rushes from higher regions. The rapid changes in the shape 



11 2 3 



Fig. I20. — Apparent paths of sun-spots at different times of the year. 



of certain sun-spots indicates the violence of this action. 

 Some of the spots are so large as to occupy millions of 

 square miles on the sun's surface. 



Viewed through a powerful telescope, the whole surface 

 of the sun seems to be coarsely motded. This mottling 

 is probably due to irregularities in the cloudy surface. 

 Lower levels are indicated by the darkened spots, and, 

 in these parts, light is lost by al>sorption through the 

 overlying atmosphere ; where, on the contrary, the light is 

 UTiusually bright, there the sun-clouds are probably unusually 



^ Penumbra, [torn, pcene, almost ; umbra, a shadow. 



=> Photosphere, from $wj, phos, light ; the light-giving sphere. 



