of reflexion and refraction. But the chief foundation, which consists
in the remark I have just made, was lacking in his demonstrations; and
for the rest he had opinions very different from mine, as may be will
appear some day if his writing has been preserved.

To come to the properties of Light. We remark first that each portion
of a wave ought to spread in such a way that its extremities lie
always between the same straight lines drawn from the luminous point.
Thus the portion BG of the wave, having the luminous point A as its
centre, will spread into the arc CE bounded by the straight lines ABC,
AGE. For although the particular waves produced by the particles
comprised within the space CAE spread also outside this space, they
yet do not concur at the same instant to compose a wave which
terminates the movement, as they do precisely at the circumference
CE, which is their common tangent.

And hence one sees the reason why light, at least if its rays are not
reflected or broken, spreads only by straight lines, so that it
illuminates no object except when the path from its source to that
object is open along such lines.

For if, for example, there were an opening BG, limited by opaque
bodies BH, GI, the wave of light which issues from the point A will
always be terminated by the straight lines AC, AE, as has just been
shown; the parts of the partial waves which spread outside the space
ACE being too feeble to produce light there.

Now, however small we make the opening BG, there is always the same
reason causing the light there to pass between straight lines; since
this opening is always large enough to contain a great number of