nly 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.

The meteorologist Kaemtz has often observed the same fact in the Alps.
Whenever this shadow was projected upon a cloud, his head appeared
surrounded by a luminous aureola.

To what action of light is this phenomenon due? Bouguer is of opinio