following figure, by supposing, as we have done, the luminous point to
be infinitely distant. Now it is easy to see, following the preceding
demonstration, that each small piece of this wave HC having arrived at
the plane AB, and there generating each one its particular wave, these
will all have, when C arrives at B, a common plane which will touch
them, namely a circle BN similar to CH; and this will be intersected
at its middle and at right angles by the same plane which likewise
intersects the circle CH and the ellipse AB.
[Illustration]
One sees also that the said spheres of the partial waves cannot have
any common tangent plane other than the circle BN; so that it will be
this plane where there will be more reflected movement than anywhere
else, and which will therefore carry on the light in continuance from
the wave CH.
I have also stated in the preceding demonstration that the movement of
the piece A of the incident wave is not able to communicate itself
beyond the plane AB, or at least not wholly. Whence it is to be
remarked that though the movement of the ethereal matter might
communicate itself partly to that of the reflecting body, this could
in nothing alter the velocity of progression of the waves, on which
the angle of reflexion depends. For a slight percussion ought to
generate waves as rapid as strong percussion in the same matter. This
comes about from the property of bodies which act as springs, of which
we have spoken above; namely that whether compressed little or much
they recoil in equal times. Equally so in every reflexion of the
light, against whatever body it may be, the angles of reflexion and
incidence ought to be equal notwithstanding that the body might be of
such a nature that it takes away a portion of the movement made by the
incident light. And experiment shows that in fact there is no polished
body the reflexion of which does not follow this rule.
But the thing to be above all remarked in our demonstration is that it
does not require that the reflecting surface should be considered as a
uniform plane, as has been supposed by all those who have tried to
explain the effects of reflexion; but only an evenness such as may be
attained by the particles of the matter of the reflecting body being
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