422 Dr. Wilhelm von Bezold on Twilight. 



While the first bright segment continues its course towards 

 the horizon, a repetition of the appearances already described is 

 preparing itself in the east. The eastern horizon again displays 

 a faint purple colour, or a second counter twilight, over which 

 a second (but very imperfectly denned) dark segment rises 

 up. In the west there appears over the first bright segment a 

 second twilight-sheen, but not reaching to quite the same eleva- 

 tion as the first ; it is separated from the bright segment by a 

 dirty yellowish-green stratum. It moves downwards in the same 

 way as the first, and a second purple light is gradually developed, 

 exactly like the first, except that it is much less intense and has 

 somewhat more of a reddish tint. On many days it is as in- 

 tense as the first is on others, though for the most part it 

 is so faint that it is impossible to observe it with any cer- 

 tainty. Out of twenty-four evenings, I saw it only eight times, 

 and I was able to determine the moment of its maximum inten- 

 sity with tolerable accuracy only on four occasions. From 

 these observations, it resulted that the depression of the sun 

 corresponding to the maximum of the first purple light was to 

 the depression corresponding to the maximum of the second in 

 a nearly constant ratio of 1 : 2*2. It will of course be understood 

 that the second purple light is accompanied by a second illumi- 

 nation of favourably-situated objects ; and thus in the Alps a 

 second afterglow may often be observed. 



The second purple light undergoes the same changes as the 

 first, and consequently occasions at last the formation of a 

 second bright segment. It was the height of this segment of 

 which Lambert* made a series of measurements one evening, but 

 erroneously regarded as the continuation of what he called the 

 first twilight-bow (D dimmer ungsbog en), that is, the limit of the 

 first dark segment. When the second bright segment sinks 

 below the horizon, the last brightness disappears, and twilight 

 is at an end. This corresponds on an average to a depression 

 of the sun of about 17°. 



The course of the phenomena, which are observable in exactly 

 the inverse order, in the morning remains in general the same. 

 The intensity and extent of the individual appearances, how- 

 ever, are very different on different days ; nevertheless twilight 

 always retains its definite character, so that its course can be 

 foreseen from the first ; or, to borrow the language of mathema- 

 tics, the law remains always the same, but the constants which 

 enter into its expression vary from day to day. 



My measurements were made with an apparatus which con- 

 sisted simply of a strip of wood, one foot in length, which carried 



* Photometria, p. 448. 



