94 LIGHT SCIENCE FOR LEISURE HOURS. 



relatively dark pores all over it, or, according to other 

 and better authorities, a surface of white-hot spots 

 spread over a relatively dark background. Here we 

 are describing merely its appearance ; what the con- 

 stitution of this surface may in reality be remains yet 

 to be determined. 



Beneath the photosphere there are vast depths of 

 vapor, for when the photosphere is broken through 

 where spots are formed, the spectroscope tells us that 

 the relatively dark regions thus disclosed are filled 

 with the vapors of various elements. "We know that 

 the dark lines which cross the rainbow-tinted solar 

 spectrum are caused by the light-absorbing action of 

 the vapors which surround the sun, and these lines 

 are seen more distinctly in the spectrum of a sun-spot 

 than in that of the photosphere. 



Now, it is worthy of notice that all that has thus far 

 been discovered tends to confirm the theroy put for- 

 ward nearly a century ago by Sir William Herschel. 

 That thoughtful observer recognized in the solar pho- 

 tosphere a widely-extended layer of luminous clouds, 

 while he regarded the light of the penumbrse of sun- 

 spots as coming from a lower cloud-layer. He con- 

 ceived that up-rushes of vapor, thrusting aside both 

 layers, caused the appearance of a solar spot. We have 

 heard a great deal lately of the English and Conti- 

 nental theories of the solar constitution ; but the 

 evidence we have recently obtained goes far to show 



