Page:Popular Science Monthly Volume 28.djvu/484

470 distinction from the objective rings, A H, A J, A N. In the same way, the particles situated on the cone P A S will contribute to the formation of a bright subjective ring of radial angle R A P. The center R will appear to be surrounded by dark and bright rings.

Now we must introduce the supposition of many-colored or poly-chromatic light—the white light of many wave-lengths that comes from the sun. Such light, passing a fine thread, forms a series of prismatic bands on a screen; passing a single particle, it forms a series of concentric prismatic rings with the blue inside; for the first blue ring will fall a little inside of the first yellow, and the first yellow inside of the first red—and so on with the others, until at a distance from the center the outer rings overlap irregularly. The subjective rings seen when white light passes through a transparent medium containing many one-sized particles will, for the same reason, appear many-colored, with the blue inside and the red outside; the central area will be white, with a reddish margin.

Finally, the actual case is reached when the suspended particles are of different sizes. The colors of the central area now overlap so irregularly that they unite to form a whitish or silvery disk; but the outer red margin of the central area formed by the smallest particles is still uncounterbalanced. The silvery disk will be reddish about the circumference; and the colors thus deduced by theory are so closely like those observed in Bishop's ring around the sun that it may be safely considered a diffraction corona. The outer rings are too faint to be seen in daytime.

Colored coronal rings may be seen around a light when looking at it through a glass strewed lightly over with spores of lycopodium; they are so nearly of the same size that a number of concentric rings appear. Kiessling describes some interesting experiments with thin artificial clouds of condensed vapors, through which the sun is seen surrounded with coronal rings. The moon is often surrounded with similar rings of small diameter, formed by diffraction, probably on small floating particles of ice, even when the sky seems clear. These are easily distinguished from halos. The latter are of definite and much larger diameter, and, when seen around the moon, are generally whitish; if formed around the sun, they are visibly colored with the red inside; and they are due to refraction and reflection on minute ice-crystals.

All this is safe enough; it is the origin of the diffracting particles and the long endurance of their effect that give trouble. Indeed, the experimental and mathematical knowledge of optics, based on the undulatory theory of light, has advanced so far that the physicist is now better able to suggest processes by which effects may be produced than the meteorologist is able to apply them. The physicist can safely say that a sufficient supply of extremely fine liquid or solid dust scattered through the atmosphere would produce just such a solar corona