composed, maintains this liquidity in spite of the pressure which
hitherto any one has been minded to apply to it.

The rarity of transparent bodies being then such as we have said, one
easily conceives that the waves might be carried on in the ethereal
matter which fills the interstices of the particles. And, moreover,
one may believe that the progression of these waves ought to be a
little slower in the interior of bodies, by reason of the small
detours which the same particles cause. In which different velocity of
light I shall show the cause of refraction to consist.

Before doing so, I will indicate the third and last mode in which
transparency may be conceived; which is by supposing that the movement
of the waves of light is transmitted indifferently both in the
particles of the ethereal matter which occupy the interstices of
bodies, and in the particles which compose them, so that the movement
passes from one to the other. And it will be seen hereafter that this
hypothesis serves excellently to explain the double refraction of
certain transparent bodies.

Should it be objected that if the particles of the ether are smaller
than those of transparent bodies (since they pass through their
intervals), it would follow that they can communicate to them but
little of their movement, it may be replied that the particles of
these bodies are in turn composed of still smaller particles, and so
it will be these secondary particles which will receive the movement
from those of the ether.

Furthermore, if the particles of transparent bodies have a recoil a
little less prompt than that of the ethereal particles, which nothing