THE 

 LONDON, EDINBURGH, and DUBLIN 



PHILOSOPHICAL MAGAZINE 



AND 



JOURNAL OF SCIENCE. 



[FOURTH SERIES.] 



SEPTEMBER 1865. 



XXI. Observations on the Polarization of the Atmosphere, made 

 at St. Andrews in 1841, 1842, 1843, 1844, and 1845. By 

 Sir David Brewster, K.H., D.C.L., F.R.S., $c. 



[Concluded from p. 129.] 



IN the year 1840, when M. Babinet had occasion to visit the 

 sea coast, he proposed to observe if the neutral point of 

 Arago varied in its height as the sun rose or set, and to observe 

 it also when the sun was beneath the horizon ; but he was 

 allured from these observations by a circumstance which he had 

 never even suspected, namely, the existence of a second neutral 

 point above the setting sun, and nearly as high in the atmo- 

 sphere as the neutral point of Arago. "I afterwards"*, he 

 says, " determined a great number of times the position of this 

 new neutral point, which appeared in the west. even when the 

 sun was just on the horizon before setting, and in the east when 

 he had risen only a few degrees. A very imperfect estimate 

 made me sometimes believe that this new neutral point was a 

 little less high than that of Arago." 



Observations on Babinet's Neutral Point, 



The following observations on this neutral point were made 

 generally on the same day, and even in the same hour, as those 

 on Arago's neutral point, and therefore under the same atmo- 

 spherical influences. 



* Comptes Rendus, &c. 1840, vol. xi. p. 619. 

 Phil Mag. S. 4. Vol. 30. No. 202. Sept. 1865. M 



