But it might here be asked: since the meeting of the wave AC against
the surface AB ought to produce movement in the matter which is on the
other side, why does no light pass there? To which the reply is easy
if one remembers what has been said before. For although it generates
an infinitude of partial waves in the matter which is at the other
side of AB, these waves never have a common tangent line (either
straight or curved) at the same moment; and so there is no line
terminating the propagation of the wave AC beyond the plane AB, nor
any place where the movement is gathered together in sufficiently
great quantity to produce light. And one will easily see the truth of
this, namely that CB being larger than 2/3 of AB, the waves excited
beyond the plane AB will have no common tangent if about the centres K
one then draws circles having radii equal to 3/2 of the lengths LB to
which they correspond. For all these circles will be enclosed in one
another and will all pass beyond the point B.
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
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