rays. It appears from the things explained above that the progression
or propagation of a small part of a wave of light is properly what one
calls a ray. Now these rays, instead of being straight as they are in
homogeneous media, ought to be curved in an atmosphere of unequal
penetrability. For they necessarily follow from the object to the eye
the line which intersects at right angles all the progressions of the
waves, as in the first figure the line AEB does, as will be shown
hereafter; and it is this line which determines what interposed bodies
would or would not hinder us from seeing the object. For although the
point of the steeple A appears raised to D, it would yet not appear to
the eye B if the tower H was between the two, because it crosses the
curve AEB. But the tower E, which is beneath this curve, does not
hinder the point A from being seen. Now according as the air near the
Earth exceeds in density that which is higher, the curvature of the
ray AEB becomes greater: so that at certain times it passes above the
summit E, which allows the point A to be perceived by the eye at B;
and at other times it is intercepted by the same tower E which hides A
from this same eye.
[Illustration]
But to demonstrate this curvature of the rays conformably to all our
preceding Theory, let us imagine that AB is a small portion of a wave
of light coming from the side C, which we may consider as a straight
line. Let us also suppose that it is perpendicular to the Horizon, the
portion B being nearer to the Earth than the portion A; and that
because the vapours are less hindering at A than at B, the particular
wave which comes from the point A spreads through a certain space AD
while the particular wave which starts from the point B spreads
through a shorter space BE; AD and BE being parallel to the Horizon.
Further, supposing the straight lines FG, HI, etc., to be drawn from
an infinitude of points in the straight line AB and to terminate on
the line DE (which is straight or may be considered as such), let the
different penetrabilities at the different heights in the air between
A and B be represented by all these lines; so that the particular
wave, originating from the point F, will spread across the space FG,
and that from the point H across the space HI, while that from the
point A spreads across the space AD.
Now if about the centres A, B, one describes the circles DK, EL, which
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