Now it is to be remarked that from the moment when the angle DAQ is
smaller than is requisite to permit the refracted ray DA to pass into
the other transparent substance, one finds that the interior reflexion
which occurs at the surface AB is much augmented in brightness, as is
easy to realize by experiment with a triangular prism; and for this
our theory can afford this reason. When the angle DAQ is still large
enough to enable the ray DA to pass, it is evident that the light from
the portion AC of the wave is collected in a minimum space when it
reaches BN. It appears also 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.