234 PHYSICS. 
rays from the sun, they all, excepting the cirrus, exhibit traces of colered 
rings. This phenomenon, although very frequent, is not always of equal 
beauty and distinctness; to observe it we may make use of the blackened 
glass plate already referred to. White clouds in the vicinity of the sun, 
with their edges parallel to the horizon, do not exhibit very well defined 
rings, but rather lively prismatic colors in the shape of streaks, ten and 
more degrees from the sun, and parallel to the horizon; these are generally 
green in the middle and surrounded by a red border. 
The so-called halo of glory, seen about the human shadow when cast on 
the grass, or upon a surface curved with dew by a declining sun, is a 
curious appearance produced by the reflection of light from the dew-drops, 
or from the cylindrical smooth stems of the straw or grass; as the entire 
lustre depends on the position of the eve, the sun, and the reflecting surface, 
each one sees only the glory about the shadow of his own head, that of 
others, even though very near, being invisible. 
Among halos is to be counted the appearance produced where the shadow: 
of the observer is cast on a cloud, his head appearing surrounded by colored 
rings. The first observation of the kind was made by Bouguer in the 
Cordilleras in the middle of the last century. He and his companions, 
standing oy the top of a high mountain, about sunrise, saw each one his 
own shadow depicted on an opposite cloud, the head surrounded by from 
three to four brilliantly colored concentric rings, the red being outside, and 
the other colors following in the order of the spectrum. Around these 
rings was another of a white color, and about 67° in diameter. Scoresby, 
in his arctic voyage, frequently observed this interesting phenomenon, 
which, according to him, always appears when sunshine and cloud co-exist, 
a circumstance which often occurs in the polar regions, the thin fog which 
there rests on the surface of the sea extending to a height of from 150 to 
180 feet. An observer at the masthead of a vessel discerns his shadow in 
the fog opposite to the sun, the head surrounded by four or five colored 
rings. Howard and several companions once observed their shadows, and 
that of the rock on which they stood, depicted on a cloud or fog bank 
sweeping below them (pl. 27, fig. 7). This phenomenon is very common 
in the Alps, particularly on the Rigi and on the Brocken, where it is called 
the Spectre of the Brocken. 
The larger circles of twenty-two to twenty-three, and forty-six to forty- 
seven degrees radius, sometimes seen about the sun and moon, are of entirely 
different character from those previously described. The colors of the 
smaller ring are generally much less lively than those of the rainbow, red, 
however, being recognisable on the inner edge, while the outer is very 
indistinct; the colors of the larger circle, which is much the rarer, exhibit 
more depth and purity. P/. 25, fig. 10, represents a simple, fig. 11 a 
double halo of this character; fig. 12 is a simple halo combined with a 
colored ring or true halo. Halos about the sun and the moon are very 
abundant, but are generally appreciable only by means of the blackened 
glass plate; Kaemtz supposed that at least sixty were visible in Germany 
each year. The general.conditions of formation are pretty much the same 
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