( 2 54 ) 
To conclude, the Canopy in acompleat Aurora Bo- 
realis he looks upon to be an Objed purely optical, a 
Ample Appearance arifing from a Angular Diftrbution 
of feveral perpendicular Columns, or Trains of Zodia- 
cal Matter^ as he explains more at large in two Fi- 
gures. This Exa&nefs and Regularity in the Diftri- 
bution makes it an uncommon Phenomenon ; fo 
that among an hundred Aurora Boreales that have 
been obferved, he has met with but three attended 
with a Corona. 
What remains, is only to take Notice of fome of 
the Queries which relate to feveral Appearances in 
Nature, that feem to be explicable by our Author's 
Hypothefes of a Solar Atmofphere, fuch as the Ne- 
bula, or lucid Spots obferv’d among the fix’d Stars, 
the Spots in the Sun, the Atmofphere and Tails of 
Comets, &c. 
The Nebula are certain luminary Spots or Patches, 
which difcover themfelves only by the Telefcope, and 
appear to the naked Eye like fmall fix’d Stars. They 
are fix in Number, and are accurately defcribed in 
Philo foph. franfaft. N° 347. Some of them have 
no Sign of a Star in the middle of them, and are 
properly Nebula t others have, and then are called 
Nebulofa. They are look'd upon by fome to be in 
reality nothing elfe but the Light coming from an 
immenfe great Space in the iEther, thro’ which a 
lucid Medium is diffufed, that Ihines with its own 
proper Luftre, making a perpetual uninterrupted Day, 
by no means owing to the Illumination of a central 
Body, or Star. . ■ - 
But Mr. Mairan feems to be of another Mind, 
and queries thus : Since the fix’d Stars are Bodies of 
the 
