48 



NATUKE FOR ITS OWN SAKE 



Trantpar- 

 ency of 

 the blue. 



Sky depth. 



particles. These reflecting particles seen in 

 mass apparently make a roof above us which 

 [ooks to be ten or fifteen miles in height. It is 

 merely an appearance, however, and our not too 

 reliable eyes deceive us. It is known that the 

 atmosphere is from two hundred to five hun- 

 dred miles in thickness, perhaps more, and 

 there is no demarcation line where the blue be- 

 gins or ends. Nor is there any point in this 

 blue where cloudiness, haziness, or opacity shows. 

 There is not a blur or film upon it, save where 

 it is influenced by earthly vapors and dust. The 

 sky itself is everywhere transparent, else we 

 should not receive light through it or see the 

 sun, moon, and stars shining beyond it. 



The recognition of sky distances is not easily 

 made by the eye. A glance upward may tell us 

 of five or fifty miles, as our imagination rather 

 than our focus is adjusted. Looking out and 

 over a tract of earth, we conceive distance 

 by perspective lines, by objects decreasing in 

 size, by the diminution of color, and the in- 

 creased thickness of atmosphere. They are all 

 optical guide-posts by which we can reckon with 

 depth and width. But no such conditions exist 

 in looking skyward. It is true we are looking 

 through thick air to thin air, and beyond that 



