THE SUN'S TRUE ATMOSPHERE. 103 



the edge of the inner globe's disc. In the former case 

 we only look though half an inch of glass, in the latter 

 we look through a much greater range of glass ; as the 

 reader will see at once if he draw two concentric circles 

 nearly equal in size to represent the inner globe and 

 its enclosing shell. It is easy to calculate how long 

 the range of glass actually is in the latter case. I have 

 just gone through the calculation, and find that when 

 the eye is directed to the edge of the enclosed globe, 

 its line of sight passes through rather more than four 

 inches and a quarter, so that more than eight times as 

 much light is absorbed as in the case where the eye 

 looks at the middle of the inner globe's disc, or directly 

 through half an inch of glass. 



Xow we cannot tell what proportion holds in the 

 case of the sun's disc, because we do not know how 

 much light has been absorbed where we look at the 

 middle of the disc. All we know is that whatever re- 

 mains after such absorption is about twice as much as 

 we receive from near the edge of the disc. It is easily 

 seen that this knowledge is insufficient for our require- 

 ments. But there can be no question whatever that 

 the total absorption near the edge exceeds many times 

 that near the middle of the disc ; and on very reason- 

 able assumptions as to this excess, it may readily be 

 shown that the absorbing atmosphere cannot exceed 

 some five or six hundred miles in depth. Probably it 

 is even shallower. 



' Now, there is a circumstance which perfectly ac- 

 counts for the non-recognition by spectroscopists of 



