NATURE FOR ITS OWN SAKE 
Sky depth. 
particles. These reflecting particles seen in 
mass apparently make a roof above us which 
looks 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 nosuch conditions exist 
in looking skyward. It is true we are looking 
through thick air to thin air, and beyond that 
