112 LIGHT SCIENCE FOR LEISURE HOURS. 



In the above estimate, I have supposed the measure- 

 ment to be made from the sun's visible surface. But 

 it is very unlikely that that surface is the true lower 

 limit of the atmosphere. It seems far more probable 

 that the surface we see is merely a layer of clouds (as 

 Sir William Herschel suggested so long ago) in the 

 solar atmosphere, and that the actual depth of the 

 atmosphere is more truly indicated by the appearances 

 seen when large sun-spots are examined. That these 

 spots are cavities has been abundantly established. 

 That they are openings through layers of solar clouds 

 has not been indeed demonstrated, yet it is difficult to 

 conceive how they can otherwise be interpreted. As 

 to the way in which the spots are formed, theorists are 

 at issue, some urging that there is an uprush from 

 depths beneath the solar surface ; others, that there is 

 a downrush of matter from without. But neither of 

 these views is in any way incompatible with Herschel's 

 theory, that the spots are openings in solar cloud- 

 layers. 



We might thus be led to compare the solar atmo- 

 sphere with our own, though it will of course be 

 obvious that there are many marked points of differ- 

 ence. But in our own atmosphere we have at least 

 two distinct cloud-levels, the region, namely, where the 

 cumulus or wool-pack clouds are formed, and that 

 where the cirrus or feathery clouds, make their ap- 

 pearance. There is air above the cirrus clouds, air be- 

 tween the cirrus and cumulus layers, and air between 

 the cumulus clouds and the earth. And precisely in 



