Chase.] 224 f^P"'- 



the 3d, 4th, and 5th, 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 the same reason, Babinet's neutral point is often better 

 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 mid- 

 day. 



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. [4] 30, 124), upon which he saw 

 all the points. 



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

 favorable atmospheric conditions the three points are, therefore, some- 

 times 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 : 



Satisfactory observations were made on 



All the neutral points were seen on 



There were no satisfactory observations on 



" '■'■ 39 observations of Arago's neutral point on 

 " « 93 " '' Babinet's " " " 



" '< 59 " '' Brewster's " " '' 



Arago's neutral point was remai'kably distinct on 



Babinet's " " '< " " " 



Brewster's " " " " " " 



Arago's was the only one observed on 



Babinet's '' '' " " " 



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



Arago's and Babinet's " "■ " " 



The three points were simultaneously visible on April 5th, from 



4" 32' to 4'' 42' p. M. 



