420 Dr. Wilhelm von Bezold on Twilight. 



relates to a phenomenon which, though of daily occurrence, has 

 been hitherto very imperfectly examined. Notwithstanding the 

 extensive literature* which we possess on the subject of Twilight, 

 there is not in the whole of it a single correct and complete de- 

 scription of the phenomenon, to say nothing of anything like a 

 satisfactory theory. I will therefore, in the first place, give a 

 short description of the occurrence of twilight, based on my own 

 observations, made on cloudless evenings, and continued for 

 almost a whole year, in Miinden and in the Alps, and will then 

 compare the results with those of the theory which has hitherto 

 been generally received, so as to show the incorrectness of the 

 latter. 



I. A short time before sunset the lower part of the sky be- 

 comes covered with a yellowish- white tint, which towards the 

 east passes into a dull purple. This purple zone, the counter- 

 twilight (Gegenddmmerung) as it is called, increases in intensity 

 about the moment of sunset, and reaches, according to the con- 

 dition of the atmosphere, from 6° to 12° above the horizon. 

 As the sun sinks, the ashy-grey shadow of the earth rises in 

 the form of a segment, in consequence of which Le Mairan called 

 it the dark segment. As soon as the limb of this segment, which, 

 for reasons afterwards stated, I will call the first dark segment, 

 has attained an elevation of more than 1° or 2°, it is tolerably 

 sharply defined. It rapidly pursues its course towards the 

 zenith, seeming to stretch itself in the manner of a dark veil 

 over the countertwilight. As it approaches the upper edge of 

 the latter, its own limit becomes more and more indistinct, until 

 at last it is impossible to recognize it any longer. At this time 

 the sun is from 3° to 4° below the horizon. 



Whilst this is taking place in the east, a very different spec- 

 tacle is displayed in the western heavens. The western horizon 

 is coloured, at sunset and afterwards, orange-yellow or even red, 

 and this yellowish coloration is defined from the upper blue por- 

 tions of the sky, at an elevation of from 9° to 12°, by a very 

 transparent, shining, whitish zone, which gradually spreads out 

 in a horizontal direction. I will call it, with Brandes, the twi- 

 light-sheen (Dammerungsschein) . 



While the twilight-sheen sinks towards the horizon, very de- 

 licate purple tints appear at an elevation of about 25°. They 

 increase rapidly in intensity, and form a circular disk with con- 

 stantly augmenting radius, whose lower part seems to be covered 

 or cut off by the yellow tint. This phenomenon, which I will 



* Nearly the whole of the earlier literature is quoted in Kamtz, Me- 

 teorologie, vol. iii. pp. 50 et seq., and in a paper by Forbes in the Edin- 

 burgh Transactions, vol. xiv. The more recent literature is referred to 

 in my paper above-mentioned. 



