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