183. In all observations, to have the true altitude of the Sun, Moon, or
Stars, the refraction must be subtracted from the observed altitude. But
the quantity of refraction is not always the same at the same altitude;
because heat diminishes the air’s refractive power and density, and cold
increases both; and therefore no one table can serve precisely for the
same place at all seasons, nor even at all times of the same day; much
less for different climates: it having been observed that the horizontal
refractions are near a third part less at the Equator than at _Paris_,
as mentioned by Dr. SMITH in the 370th remark on his Optics, where the
following account is given of an extraordinary refraction of the
sun-beams by cold. “There is a famous observation of this kind made by
some _Hollanders_ that wintered in _Nova Zembla_ in the year 1596, who
were surprised to find, that after a continual night of three months,
the Sun began to rise seventeen days sooner than according to
computation, deduced from the Altitude of the Pole observed to be 76°:
which cannot otherwise be accounted for, than by an extraordinary
quantity of refraction of the Sun’s rays, passing thro’ the cold dense
air in that climate. KEPLER computes that the Sun was almost five
degrees below the Horizon when he first appeared; and consequently the
refraction of his rays was about nine times greater than it is with us.”
184. The Sun and Moon appear of an oval figure as _FCGD_, just after
their rising, and before their setting: the reason is, that the
refraction being greater in the Horizon than at any distance above it,
the lowermost limb _G_ appears more elevated than the uppermost. But
although the refraction shortens the vertical Diameter _FG_, it has no
sensible effect on the horizontal Diameter _CD_, which is all equally
elevated. When the refraction is so small as to be imperceptible, the
Sun and Moon appear perfectly round, as _AEBF_.
[Sidenote: Our imagination cannot judge rightly of the distance of
inaccessible objects.]
185. We daily observe, that the objects which appear most distinct are
generally those which are nearest to us; and consequently, when we have
nothing but our imagination to assist us in estimating of distances,
bright objects seem nearer to us than those which are less bright, or
than the same objects do when they appear less bright and worse defined,
even though their distance in both cases be the same. And as in both
cases they are seen under the same angle[44], our imagination naturally
suggests an idea of a greater distance between us and those objects
which appear fainter and worse defined than those which appear brighter
under the same Angles; especially if they be such objects as we were
never near to, and of whose real Magnitudes we can be no judges by
sight.
[Sidenote: Nor always of those which are accessible.]
186. But, it is not only in judging of the different apparent Magnitudes
of the same objects, which are better or worse defined by their being
more or less bright, that we may be deceived: for we may make a wrong
conclusion even when we view them under equal degrees of brightness, and
under equal Angles; although they be objects whose bulks we are
generally acquainted with, such as houses or trees: for proof of which,
the two following instances may suffice.
[Sidenote: The reason assigned.