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.


§ 6. _Interference of Light_.

[Illustration: Fig. 11.]