line which is perpendicular to the horizon.
[Illustration]
Let BC be the wave which brings the light to the spectator who is at
B, and let BD be the straight line which intersects this wave at right
angles. Now because the ray or straight line by which we judge the
spot where the object appears to us is nothing else than the
perpendicular to the wave that reaches our eye, as will be understood
by what was said above, it is manifest that the point A will be
perceived as being in the line BD, and therefore higher than in fact it
is.
Similarly if the Earth be AB, and the top of the Atmosphere CD, which
probably is not a well defined spherical surface (since we know that
the air becomes rare in proportion as one ascends, for above there is
so much less of it to press down upon it), the waves of light from the
sun coming, for instance, in such a way that so long as they have not
reached the Atmosphere CD the straight line AE intersects them
perpendicularly, they ought, when they enter the Atmosphere, to
advance more quickly in elevated regions than in regions nearer to the
Earth. So that if CA is the wave which brings the light to the
spectator at A, its region C will be the furthest advanced; and the
straight line AF, which intersects this wave at right angles, and
which determines the apparent place of the Sun, will pass above the
real Sun, which will be seen along the line AE. And so it may occur
that when it ought not to be visible in the absence of vapours,
because the line AE encounters the rotundity of the Earth, it will be
perceived in the line AF by refraction. But this angle EAF is scarcely
ever more than half a degree because the attenuation of the vapours
alters the waves of light but little. Furthermore these refractions
are not altogether constant in all weathers, particularly at small
elevations of 2 or 3 degrees; which results from the different
quantity of aqueous vapours rising above the Earth.
And this same thing is the cause why at certain times a distant object
will be hidden behind another less distant one, and yet may at another
time be able to be seen, although the spot from which it is viewed is
always the same. But the reason for this effect will be still more
evident from what we are going to remark touching the curvature of
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