uld not be any body that was not transparent. For by the same
reasoning about the hollow sphere which I have employed to prove the
smallness of the density of glass and its easy penetrability by the
ethereal matter, one might also prove that the same penetrability
obtains for metals and for every other sort of body. For this sphere
being for example of silver, it is certain that it contains some of
the ethereal matter which serves for light, since this was there as
well as in the air when the opening of the sphere was closed. Yet,
being closed and placed upon a horizontal plane, it resists the
movement which one wishes to give to it, merely according to the
quantity of silver of which it is made; so that one must conclude, as
above, that the ethereal matter which is enclosed does not follow the
movement of the sphere; and that therefore silver, as well as glass,
is very easily penetrated by this matter. Some of it is therefore
present continuously and in quantities between the particles of silver
and of all other opaque bodies: and since it serves for the
propagation of light it would seem that these bodies ought also to be
transparent, which however is not the case.

Whence then, one will say, does their opacity come? Is it because the
particles which compose them are soft; that is to say, these particles
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, su