The piece C of the wave AC, will in a certain space of time advance as
far as the plane AB at B, following the straight line CB, which may be
supposed to come from the luminous centre, and which in consequence is
perpendicular to AC. Now in this same space of time the portion A of
the same wave, which has been hindered from communicating its movement
beyond the plane AB, or at least partly so, ought to have continued
its movement in the matter which is above this plane, and this along a
distance equal to CB, making its own partial spherical wave,
according to what has been said above. Which wave is here represented
by the circumference SNR, the centre of which is A, and its
semi-diameter AN equal to CB.

If one considers further the other pieces H of the wave AC, it appears
that they will not only have reached the surface AB by straight lines
HK parallel to CB, but that in addition they will have generated in
the transparent air, from the centres K, K, K, particular spherical
waves, represented here by circumferences the semi-diameters of which
are equal to KM, that is to say to the continuations of HK as far as
the line BG parallel to AC. But all these circumferences have as a
common tangent the straight line BN, namely the same which is drawn
from B as a tangent to the first of the circles, of which A is the
centre, and AN the semi-diameter equal to BC, as is easy to see.

It is then the line BN (comprised between B and the point N where the
perpendicular from the point A falls) which is as it were formed by
all these circumferences, and which terminates the movement which is
made by the reflexion of the wave AC; and it is also the place where
the movement occurs in much greater quantity than anywhere else.
Wherefore, according to that which has been explained, BN is the