136 Astronomical Notes. 



out;, became visible to the naked eye even before sunset. 

 Sirius, for the reason given above, lie thus saw only seven 

 times ; but another of his 1st mag. stars, when near the 

 zenith (he does not specify which, but from the date, June, it 

 must have been Arcturus), he actually perceived 24m. hefore 

 sunset, and gives as the mean of such observations by a prac- 

 tised eye, 8m., when the centre of the sun is still 40' above the 

 horizon.* As to the smaller stars, he found that, at a mean, 

 the sun's centre must sink respectively beneath it to 4° 18', 

 5° 4', 6° 50', 8° 52', and 11° 39', in order to admit of those 

 of the 2, 3, 4, 5, and 6 mags, becoming visible ; a much 

 smaller estimate, by 6° at an average, than that given by Wurin 

 at the beginning of this century. The final extinction of hori- 

 zontal twilight was not observed at Olnmtz from the extreme 

 rarity of suitable opportunities : at Athens its frequent obser- 

 vation presented no difficulty on that score, but was not easily 

 accomplished, from a very different reason — the interference of 

 the Zodiacal light, which is there never absent from the morn- 

 ing or evening sky, and disturbs the observation in different 

 degrees, according to its varying brightness and inclination to 

 the horizon at different times of year. It appears that the old 

 observers could not have been misled by ihe presence of this 

 phenomenon, from the very fair value, 1 8°, which they have . 

 assigned for the sun's depression at the end of twilight ; but 

 he justly thinks it extraordinary, considering the brightness 

 and singularity of the appearance, that it should have been 

 mentioned by no one before Cassini,^ and conjectures that in 

 old times it must have been either less brilliant, or else so 

 familiarly known in more southern countries that no especial 

 notice was taken of it. At the final close of twilight he found 

 the depression of the sun beneath the horizon of Athens, Dec. 



* The old story of seeing stars in the day-time from the bottoms of wells or 

 pits is fully authenticated. I believe I have met with an account, which I cannot 

 now verify, of Sirius baving been traced for some time after sunrise by an old ob- 

 server, probably Hevel. More to the purpose is Bond's statement, that he actually 

 saw that star on one occasion when "the sun was high above the horizon and 

 shining clearly ;" and he adds, " from the readiness with which the latter could 

 be discerned, in a position where much of its light must have been lost by atmos- 

 pheric extinction, I should think it possible to see a Lyra also under favourable 

 circumstances." 



f Who was the first discoverer of this phenomenon, the mystery of which 

 seems to become more and more impenetrable, or rather, who first drew attention 

 to it, seems uncertain. It has been said, but I know not on what ground, that 

 Kepler has mentioned it. It appears from Hooke's Lecture on this " Glade of 

 Light," read before the Royal Society, June 3, 1685, that Dr. Childrey's observa- 

 tion upon it, published in 1660, " was translated into French, and printed in the 

 year 1667, as appears by the Miscellanea Curiosa Academice Naturae Curiosorum ;" 

 and by that means the Advertisement was spread in France and the rest of Europe. 

 Cassini, however, published his own observation of it in 1683 as an original dis- 

 covery, and his high character affords a fair presumption that to the best of his 

 knowledge it was so. 



