hinders us from supposing, it will again follow that the progression
of the waves of light will be slower in the interior of such bodies
than it is outside in the ethereal matter.

All this I have found as most probable for the mode in which the waves
of light pass across transparent bodies. To which it must further be
added in what respect these bodies differ from those which are opaque;
and the more so since it might seem because of the easy penetration of
bodies by the ethereal matter, of which mention has been made, that
there would 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