Astronomical Notes. 137 



1st = 17°* 7; June 1 st or July 1st = 15°"3, and at a mean of the 

 whole year, = 15 0, 92. His observations at Olmiitz, though too 

 incomplete to give anything like accurate results, sufficiently 

 show that there are two maxima for the duration of twilight, cor- 

 responding with summer and winter, and two •minima, in spring 

 and autumn. The longest mean monthly interval between 

 sunset and the appearing of the minutest stars was 106*3 

 minutes, occurring in June ; the shortest, in January, 69 '5 

 min. Similar maxima and minima were found to exist at 

 Athens ; the longest interval being, however, only 99*2 min.,, 

 owing to the difference of latitude. He adopts Bauernfeind's 

 recent value of the horizontal refraction, 34 /, 3, as more accu- 

 rate than the formerly received one, 36'. The diminution 

 of starlight from passing through the atmosphere, like that 

 of the blue colour of the sky, he considered imperceptible 

 as far as 50° or 60° from the zenith; but on the shores 

 and islands of the Mediterranean days occur in which the 

 blue tint reaches so completely down to the sea line that it is 

 difficult to convince ourselves that the tone of the zenith is a 

 little deeper ; nights, too, when the stars preserve their bril- 

 liancy to 10° or 8° from the horizon. In the most transparent 

 nights of Attica, he found that Jupiter, even when 86 o- 07 

 from the zenith, rivalled Wega overhead, and Arcturus had 

 sunk to 82 0, 3 before it' was reduced to equality with a Cygni 

 near the zenith. His comparative 'values, taken at Olmiitz, 

 gave for the brightness of Jupiter 45*2 ; for Wega or Capella, 

 38*7 j for Arcturus 36*1. From the habit of such investiga- 

 tions, when first he caught sight of Canopus, the glory of those 

 skies which we never see, deep on the sea-horizon of the Isle 

 of Cerigo, he concluded that it would at the zenith have 

 exceeded Wega by two or three of the units of his scale. How 

 that scale was constructed, he has not here given us any 

 information. 



From these researches, combined with barometrical and 

 thermometrical investigations, Schmidt has deduced the height 

 of the atmosphere, as far as it is made known to us by its 

 action upon light, equal to 46 miles at Athens on January 1, 

 decreasing, though not quite regularly, to 35 miles on June 1 

 and July 1 . These values, he thinks, may probably have been 

 augmented by refraction beyond their true amount ; and he 

 intends to prosecute the subject further. It is obvious that 

 these inquiries must be materially affected by transparency 

 of sky and keenness of vision ; and when we find that within 

 the " square" of Pegasus, Schmidt could count with the naked 

 eye 102 stars at Athens, while Argelander at Bonn could detect 

 barely 30, we may form some idea both of the extraordinary 

 facilities afforded by the Southern heavens, and of the difficulty 



