Now as concerns ordinary external reflexion, that is to say which
occurs when the angle of incidence DAQ is still large enough to enable
the refracted ray to penetrate beyond the surface AB, this reflexion
should occur against the particles of the substance which touches the
transparent body on its outside. And it apparently occurs against the
particles of the air or others mingled with the ethereal particles and
larger than they. So on the other hand the external reflexion of these
bodies occurs against the particles which compose them, and which are
also larger than those of the ethereal matter, since the latter flows
in their interstices. It is true that there remains here some
difficulty in those experiments in which this interior reflexion
occurs without the particles of air being able to contribute to it, as
in vessels or tubes from which the air has been extracted.

Experience, moreover, teaches us that these two reflexions are of
nearly equal force, and that in different transparent bodies they are
so much the stronger as the refraction of these bodies is the greater.
Thus one sees manifestly that the reflexion of glass is stronger than
that of water, and that of diamond stronger than that of glass.

I will finish this theory of refraction by demonstrating a remarkable
proposition which depends on it; namely, that a ray of light in order
to go from one point to another, when these points are in different
media, is refracted in such wise at the plane surface which joins
these two media that it employs the least possible time: and exactly
the same happens in the case of reflexion against a plane surface. Mr.
Fermat was the first to propound this property of refraction, holding
with us, and directly counter to the opinion of Mr. Des Cartes, that
light passes more slowly through glass and water than through air.