METEOROLOGY. 377 



zone, above which the ordinary blue or purple skj is visible, while be- 

 low we have yellow, theu orange, and on the horizon brownish red. 



2. Opi)Osite to the above is a dark segment which is simply the ashy- 

 tinted shadow of the earth, and which is sharply distinguished from 

 the remaining portion of the heavens. 



3. A circular region of considerable diameter of a rosy-red that is 

 generally described as the purple light; this develops above the bright 

 segment a long time before sunrise or after sunset, so that the lower 

 part of this region seems to be hidden behind the bright segment. 

 The center of this region sinks rapidly after sunset, while at the same 

 time its radius increases so that finally the limit of the borders of the 

 purple region unites with that of the bright segment, the impression 

 being as if the i^urple light slipped in behind the bright segment. The 

 purple light plays the part of a very much magnified, very much diluted, 

 image of the sun ; at the time of its greatest development the general 

 brightness is increased so that objects on the earth's surface that were 

 invisible soon after sunset become again visible. The maximum of this 

 second illumination occurs in the Ali)S when the sun is four or five de- 

 grees below the horizon, at which time the center of the purple light 

 is about 18° above the horizon, while its highest point reaches up to 40° 

 or 50O. 



As soon as the purple light has completely disappeared behind the 

 bright segment there appears a second dark segment on the opposite 

 side of the sky. Soon there further develops a second bright segment 

 above the slowly sinking first one, and only with difficulty distinguished 

 therefrom ; with very clear skies one can later occasicinally also observe 

 a second purple light, and therewith a temporary increase of bright- 

 ness, so that a third illumination of objects favorably located can be ob- 

 served. [It would seem that the occurrence of two separate periods of 

 the red or rosy or purple tint, as was observed in the winter of 1883-84 

 at many places throughout the world, is therefore simply an unusual de- 

 velopment of an ordinary phenomenon. The twilight phenomena of 

 1883-84 have been specially due to the remarkable development of the 

 second purple light, which has frequently surpassed the first purple 

 light.] (I>. M. Z., I, p. 32.) 



381. Dr. G. Hellmann contributes some observations on t\^^light made 

 by himself in Spain, 1875 to 1877, and follows this by a comparison with 

 Ijrevious authorities, from allot which he deduces the following results: 



1. The depression of the sun under the horizon at the end of the as- 

 tronomical twilight is not, as has been generally assumed, constant at 

 about 18 degrees. 



2. The dei)ression showi a decided annual period, a maximum in win- 

 ter and minimum in summer. 



3. The depression IS greater in the morning than in the evening. 



4. It shows an intimate connection with the relative humidity of the 



