set near to one another; which particles are larger than those of the
ethereal matter, as will appear by what we shall say in treating of
the transparency and opacity of bodies. For the surface consisting
thus of particles put together, and the ethereal particles being
above, and smaller, it is evident that one could not demonstrate the
equality of the angles of incidence and reflexion by similitude to
that which happens to a ball thrown against a wall, of which writers
have always made use. In our way, on the other hand, the thing is
explained without difficulty. For the smallness of the particles of
quicksilver, for example, being such that one must conceive millions
of them, in the smallest visible surface proposed, arranged like a
heap of grains of sand which has been flattened as much as it is
capable of being, this surface then becomes for our purpose as even
as a polished glass is: and, although it always remains rough with
respect to the particles of the Ether it is evident that the centres
of all the particular spheres of reflexion, of which we have spoken,
are almost in one uniform plane, and that thus the common tangent can
fit to them as perfectly as is requisite for the production of light.
And this alone is requisite, in our method of demonstration, to cause
equality of the said angles without the remainder of the movement
reflected from all parts being able to produce any contrary effect.
CHAPTER III
ON REFRACTION
In the same way as the effects of Reflexion have been explained by
waves of light reflected at the surface of polished bodies, we will
explain transparency and the phenomena of refraction by waves which
spread within and across diaphanous bodies, both solids, such as
glass, and liquids, such as water, oils, etc. But in order that it may
not seem strange to suppose this passage of waves in the interior of
these bodies, I will first show that one may conceive it possible in
more than one mode.
First, then, if the ethereal matter cannot penetrate transparent
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