wave to the corresponding point of its nearest neighbor, as, for instance, from crest to crest.
Of all colors violet light has the shortest wave-length, and red the longest. Blue is next to violet, yellow next to red, and green about an average of all. The wave-length of red light is less than twice that of the violet, and yet it would take more than 30,000 of the longest waves to which the eye is sensitive to span a single inch.
Turning, now, our attention to the atmosphere, we find that at nearly all times, and everywhere within two miles of the surface, and probably much higher still, it contains, in every cubic inch, thousands of dust particles coming from fires, from plants, from the dry earth as caught up by winds, and from still other sources. Much of this dust is excessively fine and settles down with extreme slowness. It serves, as already explained, as nuclei about which the myriads of cloud droplets are formed.
In addition to this important function, extremely fine particles of dust, and even single molecules, but not the coarser portions, as shown many years ago by Lord Rayleigh, both scatter and absorb light of all colors according to the laws: (1) that the amount both of absorption and of scattering decreases in the same proportion that the fourth power of the wave-length increases; (2) that both increase with the number of particles per unit volume, and with the average square of the volume of the individual particle.
The refractive index of the air and of the foreign substances it contains, together with certain numerical terms, also enter into the complicated equations that deal quantitatively with atmospheric absorption and scattering of light. These latter facts, since they are not essential to what follows, are mentioned here only for the sake of completeness.
Now scattering and absorption, acting according to the above laws, combine to give us the colors of the sky, because sky light is only the residual, after absorption, of that portion of sunlight which was scattered by the molecules of the atmosphere and by the foreign substances floating in it.
Since, according to the first law, but little light of very long wavelength is scattered while nearly all of exceedingly short wave-length is absorbed, it follows that the light of maximum intensity, or the prevailing color, must have some intermediate wave-length. Hence the sky overhead is neither red (long wave-length) nor violet (short wavelength). Also, from the second law, we see that different parts of the sky at the same time, and the same parts of the sky at different times, will have different colors owing to the amount, aggregation and distribution of atmospheric dust.
When these particles are relatively few and small the prevailing color is blue. On the other hand, where the dust motes increase in size