98 The Sky 
small particles in the air, and exhibit a variety of colors. They 
are frequently observable as a small circle of light directly 
surrounding the sun or moon. The size of the circle of light 
permits us to compute the average size of the particles pro- 
ducing it. The circle of light is spoken of as the corona. If the 
corona decreases in size it signifies the fact that the particles 
(in this case raindrops ) are growing in size and that we may 
expect rain. If the corona grows in size it signifies that the 
drops are evaporating and that fair weather is at hand. 
SPECTERS OF THE BROCKEN 
A variation of diffraction is the Specter of the Brocken 
which may be observed when your shadow is cast upon a 
bank of fog or mist. Bright colors and pearly white light rings 
surround the dark center of the shadow. They derive their 
name from a German mountain of the same name, from 
whose summit they are frequently visible. Naturally enough, — 
they were a source of great amusement, and supernatural — 
speculation for the inhabitants of the region. It must be ad- 
mitted that they have a frightening aspect. 
NAVIGATIONAL USES 
Early navigators made considerable use of reflection of 
colors from land or sea surface upon the bottoms of clouds 
overlying them, or in the sky itself. The lighter greenish © 
color of the water in the lagoon of an atoll is reflected by 
clouds overhead. Whalers scan the sky for iceblink, a whitish 
discoloration of the sky due to the presence of ice on the sur- 
face below. In this way they are warned of the approach of © 
dangerous icebergs and fields. On the other hand, should 
they be icebound, a dark streak in the sky, water sky, may 
reveal a clear passage or open water. 
