Ether. But let us suppose that it transmits this movement less
quickly, by one-third, for instance. Movement will then be spread from
the point A, in the matter of the transparent body through a distance
equal to two-thirds of CB, making its own particular spherical wave
according to what has been said before. This wave is then represented
by the circumference SNR, the centre of which is A, and its
semi-diameter equal to two-thirds of CB. Then if one considers in
order the other pieces H of the wave AC, it appears that in the same
time that the piece C reaches B they will not only have arrived at the
surface AB along the straight lines HK parallel to CB, but that, in
addition, they will have generated in the diaphanous substance from
the centres K, partial waves, represented here by circumferences the
semi-diameters of which are equal to two-thirds of the lines KM, that
is to say, to two-thirds of the prolongations of HK down to the
straight line BG; for these semi-diameters would have been equal to
entire lengths of KM if the two transparent substances had been of the
same penetrability.

Now all these circumferences have for a common tangent the straight
line BN; namely the same line which is drawn as a tangent from the
point B to the circumference SNR which we considered first. For it is
easy to see that all the other circumferences will touch the same BN,
from B up to the point of contact N, which is the same point where AN
falls perpendicularly on BN.

It is then BN, which is formed by small arcs of these circumferences,
which terminates the movement that the wave AC has communicated within
the transparent body, and where this movement occurs in much greater
amount than anywhere else. And for that reason this line, in
accordance with what has been said more than once, is the propagation