Chase.] 196 [January. 



Mr. Chase, after explaining the mechanical laws which 

 appear to produce the differences between the European mag- 

 netic curves and the daily barometric curves delineated in 

 Leverrier's "Bulletin International," presented a statement 

 of his observations on skylight polarization at Philadelphia. 



Recent observations with a Savart polariscope having led me to 

 resuks which, while generally confirmatory, difi'er, in a few particulars, 

 from those published by Sir David Brewster (Philosophical Mag. [4] 

 30, 118 and 166 Sqq.), 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 neighborhood of the 

 solar and anti-solar points. If the polariscope is rotated from the posi- 

 tive maximum, the bands gradually diminish in brilliancy, vanishing 

 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 nega- 

 tive 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 generally 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 difiiculties 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 reflec- 

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



* There is one theoretiral nentriil point below the anti-solar point. It is prob- 

 ably never above the horizon when there is light enougli to determine its position. 



