have then made no difficulty, in meditating on these things, in
supposing that the emanation of light is accomplished with time,
seeing that in this way all its phenomena can be explained, and that
in following the contrary opinion everything is incomprehensible. For
it has always seemed tome that even Mr. Des Cartes, whose aim has been
to treat all the subjects of Physics intelligibly, and who assuredly
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. Romer 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,
for instance, were to remain always at B during 30 revolutions of this
Satellite, one would see it again emerge from the shadow after 30
times 42-1/2 hours. But the Earth having been carried along during
this time to C, increasing thus its distance from Jupiter, it follows
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