The Evolution of the Sciences 



therefore led to the conclusion that the external 

 radiant surface of the sun, called the photosphere, 

 contains solids or liquids, and as, on the other 

 hand, we are equally certain that it does not 

 form a coherent whole we can only conceive it 

 to be formed of very tenuous particles, suspended 

 in a gaseous mass, where they act in the same 

 manner as the carbon dust in a candle-flame. 



What is the thickness of the photosphere, 

 and what is there inside it? The only way to 

 get an idea of this is to look through the spots, 

 which open like windows into the interior of 

 the orb. Now the measurements made by 

 Wilson and Secchi limit the mean depth of the 

 spots to about two or three thousand kilometres, 

 an insignificant figure compared with the dimen- 

 sions of the sun; we may therefore conclude 

 that the photosphere is a mere film on the surface 

 of the sun. To determine the nature of the 

 underlying layers it would be necessary to direct 

 the spectroscope towards the centre of the 

 spots, but the complexity of the results hitherto 

 obtained defies analysis and justifies only the 

 vaguest inferences. The dimness of the spectrum 

 produced by these spots, and the presence in it 



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