Dr. Wilhelm von Bezold on Twilight. 421 



call the first purple light, attains a maximum of intensity when 

 the sun is 4° 20' below the horizon. This value was the mean 

 of twenty observations varying between 3° 40' and 4° 50' as 

 extreme limits. The radius of the disk has at this moment at- 

 tained a value of about 30°, and its centre is about 15° above 

 the horizon. The centre sinks very quickly, so that the purple- 

 light finally contracts to a narrow girdle of dull purple colour, 

 very sharply marking off the yellow portion, which, from the 

 form it thus assumes, may be called the first bright segment. 

 This limitation retains its sharpness, even after the complete 

 disappearance of the purple light, which occurs when the sun 

 is about 6° below the horizon. Measurements of the height 6 

 of the limb of this segment showed that on each day the sum 

 (o-\-6=y is nearly constant, co denoting the depression of the 

 sun. The value of 7 varied on different days from 8° to 12°. 

 This law extends even to the twilight-sheen, so that we are justi- 

 fied in denoting the yellow portion in the western heavens as 

 belonging to the bright segment from the very beginning of 

 twilight. 



While the purple light, which, it may be remarked in passing, 

 shows scarcely any trace of polarization, attains its greatest in- 

 tensity, westerly objects which immediately after sunset were 

 quite pale and colourless, appear again illuminated with a tole- 

 rably bright rose tint. This illumination is often sufficiently 

 intense to be remarked in narrow town streets where the sky is 

 scarcely visible. The Alps afford at this time the glorious 

 spectacle of the afterglow (Nachgluhen) . In the vale of Cha- 

 mouni each of these appearances has its proper name : a dis- 

 tinction is drawn between the " coloration brillante du Mont- 

 Blanc," the " teinte cadavereuse," the " coloration seconde, ou 

 la resurrection," and lastly the u extinction." 



The whole of the phenomena are, however, to be seen in the 

 plain as well ; so that there is no ground whatever for seeking 

 the cause of the afterglow on Mont Blanc, as De la Rive does, 

 in the extraordinary height of the mountain. That the pheno- 

 menon does not depend upon any mere effect of contrast, as 

 Necker supposes, is proved by the fact that it may be observed 

 even in places whence the sky is not seen, but only the illumi- 

 nated objects, such, for example, as a house on the opposite side 

 of the way seen from a room. 



The moment of complete disappearance of the purple light is 

 accompanied by a remarkable diminution of daylight. For this 

 reason, doubtless, the termination of twilight, popularly so 

 called, that is, of the portion of twilight during which common 

 occupations can be pursued without artificial light, is fixed at 

 about the same moment. 



