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
set near to one another; which particles are larger than those of the
ethereal matter, as will appear by what we shall say in treating of
the transparency and opacity of bodies. For the surface consisting
thus of particles put together, and the ethereal particles being
above, and smaller, it is evident that one could not demonstrate the
equality of the angles of incidence and reflexion by similitude to
that which happens to a ball thrown against a wall, of which writers
have always made use. In our way, on the other hand, the thing is
explained without difficulty. For the smallness of the particles of
quicksilver, for example, being such that one must conceive millions
of them, in the smallest visible surface proposed, arranged like a
heap of grains of sand which has been flattened as much as it is
capable of being, this surface then becomes for our purpose as even
as a polished glass is: and, although it always remains rough with
respect to the particles of the Ether it is evident that the centres
of all the particular spheres of reflexion, of which we have spoken,