Astronomical Notes. 135 



TWILIGHT. 



The well-known director of the Athens Observatory,, Dr. J. 

 F. Julius Schmidt, has published a valuable memoir on the 

 duration of twilight, and inquiries of a kindred nature. The 

 paper is a long one, and full of laborious computations ; but 

 some of his conclusions possess considerable interest as well as 

 novelty. The subject, which has been very little explored, and 

 has received little illustration since the imperfect observations 

 of Ptolemy and Alhazen, had attracted his attention for twenty 

 years, but his experimental investigations date chiefly from 

 February, 1856. His observations, which were very nume- 

 rous, were spread over a zone reaching from N. lat. 53 0, 5 to 

 36°* 5, but were chiefly made at Olmutz, in Germany (N. lat. 

 49° 35'-7), and Athens (N. lat. 37° 58'-3), where the purity of 

 the sky afforded far greater facility, and where the subject was 

 still further pressed upon his notice by a request from the 

 Athenian Biirgermeister in connection with the lighting of the 

 town by gas ! (What would have been the astonishment of 

 Pericles !) His memoir does not, however, treat of the ter- 

 mination of ordinary or civic twilight, when 6-rnag. stars first 

 become visible in frhe zenith, but of that of astronomical 

 twilight, when the last trace of light vanishes from the hori- 

 zon, the real night begins, and the sky over the coasts of the 

 Mediterranean is granulated or powdered over with innumer- 

 able glimpses of stars of the 6 and 7 mag., too minute to be 

 fixed by the eye. His observations were always made on the 

 zenith, under which term he obviously includes a considerable 

 extent of sky; and his conclusions affect not merely the maxima 

 and minima of duration, but the height of the atmosphere, as 

 far as its existence is thus rendered perceptible. He marked, 

 of course, the progress of twilight by the successive appear- 

 ance of stars of the first 6 mags. ; but, by the term 1st mag. 

 stars, he designates, otherwise than is usually done, the excep- 

 tionally bright ones, Wega and Capella, Sirius culminating too 

 low at Olmutz on a lighter background, and Arcturus, though 

 sometimes employed, being suspected by him of slowly variable 

 light. How he classed the smaller stars, commonly reckoned 

 as of 1st mag., does not appear. These great leaders, if 

 watched for in the ordinary way, he thinks might be detected 

 from 18m. to 70m. (probably a misprint for 20m.) after sunset; 

 but if due preparation were made) by first finding the star in a 

 telescope, and then looking along the outside of the tube as a 

 guide, he found that stars of this brilliancy would come out 

 very much earlier, though the practice was so trying to the 

 sight that his results were not numerous. They are, however, 

 very curious. He found that these large stars, thus hunted 



