of the Atmosphere. 449 



On the 26th of April 184'2, when the barometer was at 

 SO'OO, and not a cloud in the sky from morning till night*, I 

 obtained the following measures : — 



April 26, 



At 10^ 53', the maximum polarization of the sky, or the 

 rotation, was 28f °, and at 1 1'^ 46', and 3*^ 42', it was 28|°. 



On the 27th of April I observed a remarkable series of 

 phaenomena relative to this neutral point. The sky was sin- 

 gularly fine — the barometer at 30-04, and at 10^ 41' the maxi- 

 mum polarization of the sky 29|^°, the greatest that I have 

 observed. At lO*" 45', the distance of the neutral point from 

 the sun was 12° 3', and consequently about SS^° above the 

 horizon. At 12^ 12', a fog came rapidly from the sea. The 

 neutral point below the sun was driven beneath the horizon, 

 and Babinet's neutral point rose almost to the zenith. At 1^ 20' 

 the fog diminished. The neutral point below the sun reap- 

 peared near the horizon, oscillating up and down, through a 

 space of 5° or 6°, as the fog became alternately denser or 

 rarer ! 



When the sky is clear, the neutral point below the sun 

 approaches to the sun as his altitude increases, and finally 

 coincides with the sun's centre when he is in the zenith. Hence 

 it follows, that when the sun is in the zenith, the two neutral 

 points in his vicinity meet in the sun, and the system of pola- 

 rization lines becomes uniaxal. 



Were the sky sufficiently clear, we should doubtless find a 

 secondary neutral point accompanying the primary one below 

 the sun ; but in our climate there is little chance of this phae- 

 nomenon being distinctly observed. 



In his observations on the antisolar neutral pointy M. Arago 

 observed that it sometimes deviated from the plane passing 

 through the antisolar point and the eye of the observer; and 

 he justly ascribed this deviation to the influence of luminous 

 clouds situated out of this plane. The same phaenomenon 

 takes place in reference to the other neutral points, though 

 the deviation is in these cases less distinctly seen, from the 

 interference of the sun's light. But it is not merely the posi- 

 tion of the neutral point that is influenced by the intrusion of 



* The lines in the spectrum were ilKdefined, from unequal refraction in 

 the air. 



Phil. Mas. S. 3. Vol. 31. No. 210. Dec. 1847. 2 G 



