_Objects_, we often mistake their bulk by mistaking their distance,
185.
Appear bigger when seen through a fog than through clear Air, and
why, _ib._
this applied to the solution of the Horizontal Moon, 187.
_Oblique Sphere_, what, 131.
_Olympiads_, what, 323. _n._
_Orbits_ of the Planets not solid, 21.
_Orrery_ described, 434, 435, 436.