has succeeded in this better than any one before him, has said nothing
that is not full of difficulties, or even inconceivable, in dealing
with Light and its properties.

But that which I employed only as a hypothesis, has recently received
great seemingness as an established truth by the ingenious proof of
Mr. Römer which I am going here to relate, expecting him himself to
give all that is needed for its confirmation. It is founded as is the
preceding argument upon celestial observations, and proves not only
that Light takes time for its passage, but also demonstrates how much
time it takes, and that its velocity is even at least six times
greater than that which I have just stated.

For this he makes use of the Eclipses suffered by the little planets
which revolve around Jupiter, and which often enter his shadow: and
see what is his reasoning. Let A be the Sun, BCDE the annual orbit of
the Earth, F Jupiter, GN the orbit of the nearest of his Satellites,
for it is this one which is more apt for this investigation than any
of the other three, because of the quickness of its revolution. Let G
be this Satellite entering into the shadow of Jupiter, H the same
Satellite emerging from the shadow.

[Illustration]

Let it be then supposed, the Earth being at B some time before the
last quadrature, that one has seen the said Satellite emerge from the
shadow; it must needs be, if the Earth remains at the same place,
that, after 42-1/2 hours, one would again see a similar emergence,
because that is the time in which it makes the round of its orbit, and
when it would come again into opposition to the Sun. And if the Earth,