so that the wave BN becomes so much the
smaller as the angle CBA or DAQ is made less; until when the latter is
diminished to the limit indicated a little previously, this wave BN is
collected together always at one point. That is to say, that when the
piece C of the wave AC has then arrived at B, the wave BN which is the
propagation of AC is entirely reduced to the same point B. Similarly
when the piece H has reached K, the part AH is entirely reduced to the
same point K. This makes it evident that in proportion as the wave CA
comes to meet the surface AB, there occurs a great quantity of
movement along that surface; which movement ought also to spread
within the transparent body and ought to have much re-enforced the
partial waves which produce the interior reflexion against the surface
AB, according to the laws of reflexion previously explained.

And because a slight diminution of the angle of incidence DAQ causes
the wave BN, however great it was, to be reduced to zero, (for this
angle being 49 degrees 11 minutes in the glass, the angle BAN is still
11 degrees 21 minutes, and the same angle being reduced by one degree
only the angle BAN is reduced to zero, and so the wave BN reduced to a
point) thence it comes about that the interior reflexion from being
obscure becomes suddenly bright, so soon as the angle of incidence is
such that it no longer gives passage to the refraction.

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 refrac