being composed of others that are smaller, are they capable of
changing their figure on receiving the pressure of the ethereal
particles, the motion of which they thereby damp, and so hinder the
continuance of the waves of light? That cannot be: for if the
particles of the metals are soft, how is it that polished silver and
mercury reflect light so strongly? What I find to be most probable
herein, is to say that metallic bodies, which are almost the only
really opaque ones, have mixed amongst their hard particles some soft
ones; so that some serve to cause reflexion and the others to hinder
transparency; while, on the other hand, transparent bodies contain
only hard particles which have the faculty of recoil, and serve
together with those of the ethereal matter for the propagation of the
waves of light, as has been said.

[Illustration]

Let us pass now to the explanation of the effects of Refraction,
assuming, as we have done, the passage of waves of light through
transparent bodies, and the diminution of velocity which these same
waves suffer in them.

The chief property of Refraction is that a ray of light, such as AB,
being in the air, and falling obliquely upon the polished surface of a
transparent body, such as FG, is broken at the point of incidence B,
in such a way that with the straight line DBE which cuts the surface
perpendicularly it makes an angle CBE less than ABD which it made with
the same perpendicular when in the air. And the measure of these
angles is found by describing, about the point B, a circle which cuts
the radii AB, BC. For the perpendiculars AD, CE, let fall from the
points of intersection upon the straight line DE, which are called the