another; and that the spreading of this movement, taking place equally
rapidly on all sides, ought to form spherical surfaces ever enlarging
and which strike our ears. Now there is no doubt at all that light
also comes from the luminous body to our eyes by some movement
impressed on the matter which is between the two; since, as we have
already seen, it cannot be by the transport of a body which passes
from one to the other. If, in addition, light takes time for its
passage--which we are now going to examine--it will follow that this
movement, impressed on the intervening matter, is successive; and
consequently it spreads, as Sound does, by spherical surfaces and
waves: for I call them waves from their resemblance to those which are
seen to be formed in water when a stone is thrown into it, and which
present a successive spreading as circles, though these arise from
another cause, and are only in a flat surface.
To see then whether the spreading of light takes time, let us consider
first whether there are any facts of experience which can convince us
to the contrary. As to those which can be made here on the Earth, by
striking lights at great distances, although they prove that light
takes no sensible time to pass over these distances, one may say with
good reason that they are too small, and that the only conclusion to
be drawn from them is that the passage of light is extremely rapid.
Mr. Des Cartes, who was of opinion that it is instantaneous, founded
his views, not without reason, upon a better basis of experience,
drawn from the Eclipses of the Moon; which, nevertheless, as I shall
show, is not at all convincing. I will set it forth, in a way a little
different from his, in order to make the conclusion more
comprehensible.
[Illustration]
Let A be the place of the sun, BD a part of the orbit or annual path
of the Earth: ABC a straight line which I suppose to meet the orbit of
the Moon, which is represented by the circle CD, at C.
Now if light requires time, for example one hour, to traverse the
space which is between the Earth and the Moon, it will follow that the
Earth having arrived at B, the shadow which it casts, or the
interruption of the light, will not yet have arrived at the point C,
but will only arrive there an hour after. It will then be one hour
12