onsensus of evidence is one of the strongest points of the
undulatory theory. The shortest waves of the visible spectrum are
those of the extreme violet; the longest, those of the extreme red;
while the other colours are of intermediate pitch or wavelength. The
length of a wave of the extreme red is such, that it would require
39,000 such waves, placed end to end, to cover one inch, while 64,631
of the extreme violet waves would be required to span the same
distance.

Now, the velocity of light, in round numbers, is 186,000 miles per
second. Reducing this to inches, and multiplying the number thus found
by 39,000, we find the number of waves of the extreme red, in 186,000
miles, to be four hundred and sixty millions of millions. _All these
waves enter the eye, and strike the retina at the back of the eye in
one second_. In a similar manner, it may be found that the number of
shocks corresponding to the impression of violet is six hundred and
seventy-eight millions of millions.

All space is filled with matter oscillating at such rates. From every
star waves of these dimensions move, with the velocity of light, like
spherical shells in all directions. And in ether, just as in water,
the motion of every particle is the algebraic sum of all the separate
motions imparted to it. One motion does not blot out the other; or, if
extinction occur at one point, it is strictly atoned for, by augmented
motion, at some other point. Every star declares by its light its
undamaged individuality, as if it alone had sent its thrills through
space.

Sec. 6. _Interference of Light_.

[Illustration: Fig. 11.]

The principle of interference, as just stated, applies to the waves of
light as it does to the waves of water and the waves of sound. And the
conditions of interference are the same in all three. If two series of
light-waves of the same length start at the same moment from a common
origin (say A, fig. 11), crest coincides with crest, sinus with sinus,
and the two systems blend together to a single system (A _m_ _n_) of
double amplitude. If both series start at the same moment, one of them
being, at starting, a whole wavelength in advance of the other, they
also add themselves together, and we have an augmented luminous
effect. The same occurs when the one system of waves is any _even_
number of semi-undulations in advance of the other. But if the one
system be half a wave-length (as at A' _a_', fig. 12), or any _odd_
number of