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
particles of the ethereal matter, which are of an inconceivable
smallness; so that it appears that each little portion of the wave
necessarily advances following the straight line which comes from the
luminous point. Thus then we may take the rays of light as if they
were straight lines.

It appears, moreover, by what has been remarked touching the
feebleness of the particular waves, that it is not needful that all
the particles of the Ether should be equal amongst themselves, though
equality is more apt for the propagation of the movement. For it is
true that inequality will cause a particle by pushing against another
larger one to strive to recoil with a part of its movement; but it
will thereby merely generate backwards towards the luminous point some
partial waves incapable of causing light, and not a wave compounded of
many as CE was.

Another property of waves of light, and one of the most marvellous,
is that when some of them come from different or even from opposing
sides, they produce their effect across one another without any
hindrance. Whence also it comes about that a number of spectators may
view different objects at the same time through the same opening, and
that two persons can at the same time see one another's eyes. Now
according to the explanation which has been given of the action of
light, how the waves do not destroy nor interrupt one another when
they cross one another, these effects which I have just mentioned are
easily conceived. But in my judgement they are not at all easy to
explain according to the views of Mr. Des Cartes, who makes Light to
consist in a continuous pressure merely tending to movement. For this
pressure not being able to act from two opposite sides at the same
time, against bodies which have no inclination to approach one
another, it is impossible so to understand what I have been saying
about two persons mutually seeing one another's eyes, or how two
torches can illuminate one another.

CHAPTER II

ON REFLEXION

Having explained the effects of waves of light which spread in a
homogeneous matter, we will examine next that whi