Sometimes these spectres are surrounded by colored concentric arcs.  Since the beginning of the present century, treatises on meteorology designate, under the name of the Ulloa circle, the pale external arch which surrounds the phenomenon, and this same circle has sometimes been called the “white rainbow.”  But it is not formed at the same angular distance as the rainbow, and, although pale, it often envelops a series of interior colored arcs.

[Illustration:  “THE SPECTRE OF THE BROCKEN”]

Ulloa, being in company with six fellow-travellers upon the Pambamarca at daybreak one morning, observed that the summit of the mountain was entirely covered with thick clouds, and that the sun, when it rose, dissipated them, leaving only in their stead light vapors, which it was almost impossible to distinguish.  Suddenly, in the opposite direction to where the sun was rising, “each of the travellers beheld, at about seventy feet from where he was standing, his own image reflected in the air as in a mirror.  The image was in the centre of three rainbows of different colors, and surrounded at a certain distance by a fourth bow with only one color.  The inside color of each bow was carnation or red, the next shade was violet, the third yellow, the fourth straw color, the last green.  All these bows were perpendicular to the horizon; they moved in the direction of, and followed, the image of the person they enveloped as with a glory.”  The most remarkable point was that, although the seven spectators were standing in a group, each person only saw the phenomenon in regard to his own person, and was disposed to disbelieve that it was repeated in respect to his companions.  The extent of the bows increased continually and in proportion to the height of the sun; at the same time their colors faded away, the spectre became paler and more indistinct, and finally the phenomenon disappeared altogether.  At the first appearance the shape of the bows was oval, but toward the end they became quite circular.  The same apparition was observed in the polar regions by Scoresby, and described by him.  He states that the phenomenon appears whenever there is mist and at the same time shining sun.  In the polar seas, whenever a rather thick mist rises over the ocean, an observer, placed on the mast, sees one or several circles upon the mist.

[Illustration:  THE ULLOA CIRCLE.]

These circles are concentric, and their common centre is in the straight line joining the eye of the observer to the sun, and extended from the sun toward the mist.  The number of circles varies from one to five; they are particularly numerous and well colored when the sun is very brilliant and the mist thick and low.  On July 23, 1821, Scoresby saw four concentric circles around his head.  The colors of the first and of the second were very well defined; those of the third, only visible at intervals, were very faint, and the fourth only showed a slight greenish tint.