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XI. Intelligence and Miscellaneous Articles, 



OBSERVATIONS ON SKYLTGHT POLARIZATION AT PHILADELPHIA. 

 BY MR. CHASE. 



T3ECENT observations with a Savart polariscope having led me to 

 -*- *• results which, while generally confirmatory, differ in a few par- 

 ticulars from those published by Sir David Brewster (Philosophical 

 Magazine, S. 4. vol. xxx. pp. 118 and 166 et seq.), I place some of 

 them on record to facilitate a comparison with similar observations 

 at other places. 



1. In all the great circles which pass through the sun the polari- 

 zation of a clear sky is positive, except in the neighbourhood of the 

 solar and antisolar points. If the polariscope is rotated from the 

 positive maximum, the bands gradually diminish in brilliancy, vanish- 

 ing at about 45°, and attaining a negative maximum at about 90°. 



2. Within the primary lemniscates, of which the solar and anti- 

 solar points are the respective centres, and the neutral points (actual 

 or theoretical*) are the limits, the polarization of a clear sky is 

 negative when the bands pass towards the sun's centre, vanishing 

 when the bands are inclined 45° to the solar radii, and attaining a 

 positive maximum when the inclination reaches 90°. 



3. Arago's and Babinet's neutral points can be seen as well before 

 sunrise as after sunset, provided the atmospheric conditions are the 

 same. Brewster gives the preference to the evening observation, 

 but apparently for no other reason than that the sky is then gene- 

 rally clearer than in the morning. (Vol. cit. p. 118.) 



4. I have repeatedly, and with little comparative difficulty, observed 

 Brewster's neutral point. In the majority of cases, when the sun's 

 altitude has been sufficient, I have been able to fix its position with 

 nearly as much facility as that of Babinet's. (For the difficulties of 

 Brewster and Babinet, see loc. cit. pp. 119, 166, 181.) 



5. Within the solar primary lemniscate it is frequently difficult 

 to make any ordinary observation of the polarized bands, on account 

 of the dazzling intensity of the light. But when the direct rays of 

 the sun have been shut off by a thin disk (placed with its edge to- 

 wards the eye, so that the polarization will not be affected by re- 

 flexion from the surface of the disk), I have often been able to mark 

 the opposite polarizations and the position of the neutral points with 

 perfect ease, even at midday. 



6. In our climate it is by no means unusual to have days on which 

 all the three neutral points can be observed, and their places deter- 

 mined. During the whole period of Brewster's observations at St. 

 Andrews he found but two such days, April 5 and 8, 1842 (loc. cit. 

 pp. 124, 163). 



7. Quasi neutral lines, dividing bands of opposite polarization, can 

 be found in nearly all parts of the sky by rotating the polariscope 

 45° from the line of maximum positive or negative polarization. 



* There is one theoretical neutral point below the antisolar point. It is 

 probably never above the horizon when there is light enough to determine 

 its position. - 



