Page:Popular Science Monthly Volume 28.djvu/482

468 rosy or purplish-pink, and at last the pure blue of the sky. The colors were wonderfully vivid for the time of day, although, of course, not so brilliant as those of a well-developed sunset; but it unfortunately seems to have very generally passed unnoticed. Inquiry among my neighbors failed to discover any one who had seen it.

Numerous observations in many parts of Europe and this country leave little room for question that the corona is produced in the upper atmosphere, and that it was continuously present above the cloudy or dusty lower air over a large part if not over the whole of the earth throughout 1884 and 1885.

The explanation of the optical process by which such a corona may be produced offers no particular difficulty. It is a relatively simple effect of diffraction, an effect of the same nature as that seen in the colored rings surrounding a light looked at through a glass that is faintly frosted over, as may be noticed almost any cold winter evening when looking out of a window. A brief statement of the process may be made, following the explanation given by Kiessling, to whom the author is much indebted in the preparation of this article.

Let us first consider the action of a beam of parallel rays of mono-chromatic light—that is, of strictly one-colored light, whose waves all agree in their period of vibration—as it passes an excessively fine thread stretched at right angles to its path, and falls on a screen beyond. The waves will be turned aside from and bent around both sides of the thread, as if diverging there from new centers of radiation. This is diffraction. A gross figure of the process is here given (Fig. 1) on a plane at right angles to the thread, T H. The point A



on the screen will be illuminated, although it is behind the thread, for the waves that reach A from either side of the thread agree in phase. Take a point, B or C, such that the distance B H exceeds B T by half a wave-length. Then the diffracted waves which agreed in phase at