these waves can be
infinitely feeble only as compared with the wave DCF, to the
composition of which all the others contribute by the part of their
surface which is most distant from the centre A.

One sees, in addition, that the wave DCF is determined by the
distance attained in a certain space of time by the movement which
started from the point A; there being no movement beyond this wave,
though there will be in the space which it encloses, namely in parts
of the particular waves, those parts which do not touch the sphere
DCF. And all this ought not to seem fraught with too much minuteness
or subtlety, since we shall see in the sequel that all the properties
of Light, and everything pertaining to its reflexion and its
refraction, can be explained in principle by this means. This is a
matter which has been quite unknown to those who hitherto have begun
to consider the waves of light, amongst whom are Mr. Hooke in his
_Micrographia_, and Father Pardies, who, in a treatise of which he let
me see a portion, and which he was unable to complete as he died
shortly afterward, had undertaken to prove by these waves the effects
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,