The pitch of sound is wholly determined by the rapidity of the vibration, as the intensity is by the amplitude.  What pitch is to the ear in acoustics, colour is to the eye in the undulatory theory of light.  Though never seen, the lengths of the waves of light have been determined.  Their existence is proved by their effects, and from their effects also their lengths may be accurately deduced.  This may, moreover, be done in many ways, and, when the different determinations are compared, the strictest harmony is found to exist between them.  This consensus 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.