128 Sir David Brewster on the Polarisation of the Atmosphere. 



On a Secondary Neutral Point accompanying Arago's 

 Neutral Point. 



When the sea horizon was terminated by a dark purple belt 

 about 1^° above it, I observed that the vertical bands of the 

 polariscope became brighter over that belt. 



The same phenomenon was seen, but less distinctly, over the 

 land horizon. It was difficult to measure the amount of this 

 new polarizing influence, but it was obvious that we should 

 observe it separately when the neutral point came above the 

 belt. In this case it would eclipse, as it were, the neutral point, 

 which would recover itself when it emerged from the belt. It 

 was obvious also that when the negative or oppositely polarized 

 bands came over the belt, the new polarizing ^influence would 

 extinguish them where they had the same polarizing force, and 

 form a secondary neutral point, the primary one being then out 

 of the belt. 



On the 8th of June, 1841, at 5 h 50 m p.m., when the polarized 

 bands were strongest, both on the land and sea horizon, 1 watched 

 the rise of the neutral point, which, as I had foreseen, did not 

 appear first in the horizon, but about \\° above it, the compensa- 

 tion taking place where the vertical or positive polarization was 

 weaker than in the horizon. We had now the singular pheno- 

 menon of a neutral point with positive polarization on each side 

 of it. When this phenomenon was more fully developed under 

 a favourable state of the horizon, the positive polarization was 

 overcome by the advancing negative polarization. The negative 

 polarization was then immediately below the ascending neutral 



¥\cr. 3. 



11 



■0:^<$0i 



point ; but at a certain distance (a few 

 degrees below the neutral point) the 

 negative polarization was compensated 

 by the excess of positive polarization 

 close to the horizon, and the beautiful i 

 phenomenon was seen of two neutral 

 points, a primary and a secondary, se- 

 parated by bands of negative polariza- 

 tion, as shown in the annexed figure. * 



1841, June 10, 6 h 40 m .— The neu- 

 tral point a little above the horizon, with 

 vertical or + polarization on both sides of 

 it. The new vertical polarization had £ 

 more than compensated the horizontal 



or negative polarization, and left a balance of positive polariza- 

 tion, which soon disappeared when the rising horizontal polari- 

 zation overpowered it. 



