and finally filling the second slit. When each wave passes through the
latter it not only pursues its direct course to the retina, but
diverges right and left, tending to throw into motion the entire mass
of the ether behind the slit. In fact, as already explained, _every
point of the wave which fills the slit is itself a centre of a new
wave system which is transmitted in all directions through the ether
behind the slit_. This is the celebrated principle of Huyghens: we
have now to examine how these secondary waves act upon each other.

[Illustration: Fig. 18.]

Let us first regard the central band of the series. Let AP (fig. 18)
be the width of the aperture held before the eye, grossly exaggerated
of course, and let the dots across the aperture represent ether
particles, all in the same phase of vibration. Let E T represent a
portion of the retina. From O, in the centre of the slit, let a
perpendicular O R be imagined drawn upon the retina. The motion
communicated to the point R will then be the sum of all the motions
emanating in this direction from the ether particles in the slit.
Considering the extreme narrowness of the aperture, we may, without
sensible error, regard all points of the wave A P as equally distant
from R. No one of the partial waves lags sensibly behind the others:
hence, at R, and in its immediate neighbourhood, we have no sensible
reduction of the light by interference. This undiminished light
produces the brilliant central band of the series.

Let us now consider those waves which diverge laterally behind the
second slit. In this case the waves from the two sides of the slit
have, in order to converge upon the retina, to pass over unequal
distances. Let A P (fig. 19) represent, as before, the width of the