[ 156 ] 



XXI. Intelligence and Miscellaneous Articles, 



on the comparative visibility of arago's, babinet's, and 



brewster's neutral points, by mr. chase. 

 TN my communication of January 5, I stated that when Brewster's 

 -*- neutral point is above the horizon, I had frequently determined its 

 position with great ease. My experience was so different from those 

 of Brewster and Babinet, that I commenced on the 6th of March 

 a series of comparative observations upon the three neutral points. 

 The month which has just ended appears to warrant the following 

 conclusions for stations in Philadelphia and its vicinity. The first, 

 second, and sixth seem to be confirmed by observations elsewhere, 

 while the third, fourth, and fifth, which are perhaps dependent 

 partly upon local atmospheric peculiarities, have never, so far as I 

 am aware, hitherto been noticed. 



1. Arago's neutral point often assumes a distinctness which is 

 never exhibited by either of the others, merely because the polarized 

 bands in the vicinity of the sun are obscured by the dazzling bril- 

 liancy of its rays. 



2. For thesame reason, Babinet'sneutralpointis oftenbetter defined in 

 the morning and evening than Brewster's during the middle of the day. 



3. But when Brewster's and Babinet's neutral points are both 

 above the horizon, if the sky is clear, the former is generally more 

 easily posited than the latter. This is especially the case at midday. 



4. On every clear day, and on a large portion of the days which 

 are partially obscured by clouds, the position of each of the neutral 

 points can be determined. Brewster records but two days during 

 five years' observations (Phil. Mag. S. 4. vol. xxx. p. 124), upon 

 which he saw all the points. 



5. Arago's neutral point often rises before Brewster's sets. Under 

 favourable atmospheric conditions the three points are, therefore, 

 sometimes simultaneously visible. 



6. Halos and clouds are frequently discernible through the polari- 

 scope which are invisible to the naked eye. 



The following abstract embodies some of the results of the month's 



observations : — Days. 



Satisfactory observations were made on 25 



All the neutral points were seen on 17 



There were no satisfactory observations on 6 



,, 39 observations of Arago's neutral point on. . 23 



93 „ Babinet's „ „ . . 22 



59 ,, Brewster's ,, „ . . 20 



Arago's neutral point was remarkably distinct on 4 



Babinet's ,, „ ,, 10 



Brewster's „ ,, ,, 11 



Arago's was the only one observed on 2 



Babinet's ,, ,, 1 



Babinet's and Brewster's the only ones seen on 1 



Arago's and Babinet's ,, „ 2 



The three points were simultaneously visible on April 5, from 

 4 h 32 m to 4 h 42 m p.m. 



