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
represent the spreading of the waves which originate from these two
points, and if one draws the straight line KL which touches these two
circles, it is easy to see that this same line will be the common
tangent to all the other circles drawn about the centres F, H, etc.;
and that all the points of contact will fall within that part of this
line which is comprised between the perpendiculars AK, BL. Then it