424 Dr. Wilhelm von Bezold on Twilight. 



moment of the complete disappearance of the first purple light 

 must be taken as the beginning of the second twilight ; and this 

 takes place nearly at the middle of the first, for the first bright 

 segment does not set until the sun is from about 9 to 12 degrees 

 below the horizon. 



I will content myself with having, on the one hand, esta- 

 blished the course of the phenomena, and having, on the other 

 hand, demonstrated that the theory hitherto accepted is alto- 

 gether incorrect and insufficient, without venturing to try to 

 replace it by a better. I believe, nevertheless, that the beha- 

 viour of the first dark and bright segments is quite capable of 

 explanation when the problem is regarded from the photo- 

 metric point of view. The brightness of any portion of the sky 

 must in fact depend, first, upon the number of illuminated 

 particles which lie upon the straight line drawn from the ob- 

 server's eye to the point in question ; secondly, upon the ab- 

 sorption of light by the atmosphere ; and lastly, upon various 

 other conditious of more or less importance. If, in the mean- 

 time, we consider only the two first-named conditions, it is easy 

 to see that, after sunset, there must be two zones with a maxi- 

 mum of brightness, the lower limit of the countertwilight and 

 the twilight-sheen. According to this, the latter w T ould mark 

 the limit between those points of the sky which send us chiefly 

 transmitted light and those which send us chiefly reflected light. 



The difference of colour which may be remarked on the two 

 sides of this zone is a confirmation of the truth of this view. In 

 like manner the course of the phenomena, as it results from 

 measurements, accords in general entirely with the conception 

 here indicated. 



The above are the most important results of an investigation 

 which I am unfortunately unable to pursue, from want of a proper 

 place for making observations. 



Briefly stated, they are as follows : — 



Twilight consists of a first and a second period, each of which 

 exhibits the dark and the bright segment with the twilight-sheen 

 and the purple light. 



These two periods of twilight overlap each other, so that the 

 beginning of the second coincides approximately with the end of 

 the first half of the first period. 



In the morning the same phenomena may be observed, but 

 they follow each other in the inverse order. 



