216 Mr. Bowdoin on amall farroumdiiy Orb, 
- If thefe obfervations have arly foundation, neither the atmof« 
phere nor its vapour, aflifted by, or afüfting, the direct light of 
the fun and ftars, can be the caufé of the phenomenon. 
- 'Theatmofphere, however, or its finer and tran{patent vapour, 
contributes to the brighter hue of the phenomenon by day : 
which may be thus explained. 
The fun’s light in its mixtftate, reflected by the atmofphere, 
or by the tranfparent vapour floating in it, enters the eye.at the 
e time wi blue ie; expanfe ; ‘and ‘both to- 
gether delineate on the retina an image, formed by-their united 
fays, each producing its effe&. ‘The light from the expan’ 
exhibiting the blue image ; ‘the light from the fun iluminat- 
-ing or brightening the image ; and both together ‘impreffing 
ithe idea of that phenomenon, as it is difplayed in a clear day. 
If it fhould be afked, from whence the concave expanfe de- 
rives its light, theanfwer is—from the numberlefs planetary or 
folar fyftems, which it includes : and particularly from thofe 
in the neighbourhood of it, which dire&ly anfwer the purpofe 
-Of enlightening, and. in other tefpedts, accommodating its in- 
"habitants, | 
~ This light, tranfinitted to the expanfethrough its atmofphere, 
is reflected back directly and indire&ly to the..fyftems from 
‘Which it iffued : to'be again, in a due fucceffion, femitted to, 
and reflected from, the expanfe. By:fach a reciprocation, and 
mutual interchange of light with each other, and among them- 
‘felves, the feveral parts may be fupplied with the quantity they 
had refpeétively emitted ; and the equilibrium -of the whole 
‘maintained ; whereby the evils, that might otherwife enfue 
from the wafte, or undue diftribution of its matter, and the 
econfequent alteration of its gravitation, might be prevented. 
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