minated by a candle flame, instead of the more intense electric
light, or when a distant platinum wire raised to a white heat by an
electric current is employed, substantially the same effects are
observed.

[Illustration: Fig. 16.]

Sec. 11. _Application of the Wave-theory to the Phenomena of
Diffraction_.

Of these and of a multitude of similar effects the Emission Theory is
incompetent to offer any satisfactory explanation. Let us see how they
are accounted for by the Theory of Undulation.

And here, with the view of reaching absolute clearness, I must make an
appeal to that faculty the importance of which I have dwelt upon so
earnestly here and elsewhere--the faculty of imagination. Figure
yourself upon the sea-shore, with a well-formed wave advancing. Take a
line of particles along the front of the wave, all at the same
distance below the crest; they are all rising in the same manner and
at the same rate. Take a similar line of particles on the back of the
wave, they are all falling in the same manner and at the same rate.
Take a line of particles along the crest, they are all in the same
condition as regards the motion of the wave. The same is true for a
line of particles along the furrow of the wave.

The particles referred to in each of these cases respectively, being
in the same condition as regards the motion of the wave, are said to
be in the same _phase_ of vibration. But if you compare a particle on
the front of the wave with one at the back; or, more generally, if you
compare together any two particles not occupying the same position in
the wave, their conditions of motion not being the same, they are said
to be in different phases of vibration. If one of the particles lie
upon the crest, and the other on the furrow of the wave, then, as one
is about to rise and the other about to fall, they are said to be in
_opposite_ phases of vibration.

There is still another point to be cleared up--and it is one of the
utmost importance as regards our present subject. Let O (fig. 17) be a
spot in still water which, when disturbed, produces a series of
circular waves: the disturbance necessary to produce these waves is
simply an oscillation up and down of the water at O. Let _m_ _n_ be
the position of the ridge of one of the waves at any moment, and _m'_
_n'_ its position a second or two afterwards. Now every particle of
water, as the wave passes it, oscillates, as we have learned, up and
down. If, then, thi