ent 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
bodies at all, their own particles would be able to communicate
successively the movement of the waves, the same as do those of the
Ether, supposing that, like those, they are of a nature to act as a
spring. And this is easy to conceive as regards water and other
transparent liquids, they being composed of detached particles. But it
may seem more difficult as regards glass and other transparent and
hard bodies, because their solidity does not seem to permit them to
receive movement except in their whole mass at the same time. This,
however, is not necessary because this solidity is not such as it
appears to us, it being probable rather that these bodies are composed
of particles merely placed close to one another and held