448 Sir David Brewster on the Polarization 



sky, with a secondary neutral point; but I have never had an 

 opportunity of observing M. Babinet's neutral point when it 

 either rose above or set beneath the horizon, which, though 

 not essential, is the most favourable for observing a secondary 

 neutral point. 



IV. On the Neutral Point below the Sutz. 



This neutral point is, as we have previously noticed, much 

 more difficult to be seen than that of M. Babinet. In No- 

 vember, December and January, it cannot be seen in our lati- 

 tudes, unless when, early in November and late in January, a 

 higher degree of polarization in the sky brings it above the 

 horizon at noon. 



As theory indicated the existence of this neutral point, I 

 long sought for it in vain ; but when I was assured of its ex- 

 istence by the discovery of negative polarization, which often 

 extended from the sun to thehorizon even when the sun's altitude 

 was 30°, I took such precautions for excluding all unnecessary 

 light from the eye that I at last observed it near the horizon, 

 with a small portion of positively polarized light beneath it. 

 I afterwards observed it repeatedly when the sun had higher 

 altitudes, and was able to measure its varying distance from 

 that luminary. On the 18th of February 1842, at noon, when 

 the sun's altitude was about 22°, I observed this neutral point 

 in the most distinct manner, the polarized bands being nega- 

 tive below the sun, and positive near the horizon. Its distance 

 from the sun, therefore, was about 15° or 16°. I afterwards 

 obtained the following measures of its distance from the sun : — 



Distance of neutral point 



h , 



On the 20th of April, in a very fine day, the wind being 

 west and the barometer 30*02, I obtained the following mea- 

 sures: — 



April 20, 



The maximum polarization of the sky at the time of these 

 observations was equal to a rotation of 25^^°, about 4^° below 

 the greatest maximum. 



