The last group of optical phenomena that we shall consider consists of those due to the process called diffraction, which occurs when light is bent around objects in its path, instead of passing through them, as in refraction. The process involves separation of the prismatic colors. The diffraction phenomena of the atmosphere are produced by the water drops of clouds and fog, or sometimes by fine dust.
Everybody is familiar with the nocturnal spectacle which Tennyson describes as
... the tender amber round
Which the moon about her spreadeth,
Moving thro’ a fleecy night.
This diffuse reddish or rainbow-tinted circle is called a corona. It occurs about the sun as well as the moon (though not easy to see on account of the glaring brightness of the luminary), and also about street lamps and other terrestrial lights when viewed through a misty atmosphere. Unlike the halos, it has no definite angular size. It is usually only a few degrees in radius. Small coronas are produced by large water drops and large coronas by small drops, while the largest of all coronas, known as Bishop’s ring, is due to exceedingly fine dust in the atmosphere, and has been seen after great volcanic eruptions.
In its commonest form the corona consists of a brownish-red ring, which, together with the bluish-white inner field between the ring and the luminary, forms the so-called aureole. If other colors are distinguishable, they follow the brownish red of the aureole (in the direction away from the luminary) in the order from violet to red; the reverse of the order seen in halos. Sometimes the sequence of colors is repeated three or four times.
Patches and fringes of iridescence are sometimes seen in the clouds at a greater distance from the luminary than that of the ordinary corona. Probably they are fragments of coronas of unusual size produced by exceedingly fine cloud particles.
Similar in appearance to the corona is the glory; a series of concentric colored rings seen around the shadow of the observer, or of his head only, cast upon a cloud or fog bank. Such a shadow, with or without the glory, constitutes the specter of the Brocken, often seen from mountain tops and from aircraft. The colored circles are sometimes called Ulloa’s rings, from the name of a Spanish savant who observed the phenomenon among the mountains of South America in the eighteenth century and has left us a vivid description of it.
The Brocken specter, though it owes its name to legends associated with the famous German mountain where witches were once believed to assemble on Walpurgis Night, is actually less frequently witnessed there than in many other parts of the world. Whenever the sun is low on one side of a mountain and a wall of mist arises from a near-by valley on the other, the mountaineer is likely to see his shadow upon the mist. If the latter consists of fine droplets of approximately uniform size, the colored rings will probably appear, and occasionally there is also a white fogbow outside of the glory. As all shadows cast by the sun taper rapidly (on account of the angular breadth of the solar disk), a well-defined Brocken specter can never be more than a few yards away from the observer. Its distance is, however, commonly overestimated—some observers have supposed it to be miles away!—and hence the erroneous idea prevails that the specters are of enormous size.
Rarely from a favorable point of vantage on a mountain, and very frequently from aircraft, the specter, instead of being seen on a vertical wall of mist when the sun is low, appears on a horizontal sheet of cloud below the observer when the sun is high. The aeronaut may thus observe the complete outline of his balloon or aeroplane, encircled with the rainbow tints of the glory. During the World War the appearance of the luminous rings was likened to the emblem painted on the wings of the Allied aeroplanes and was regarded by superstitious aviators as an omen favorable to their cause.
The glory is due to the light that is reflected back to the observer after penetrating the cloud or fog a little way and is diffracted by the superficial layer of drops in emerging.