ON THE POLARISATION OF THE ATMOSPHERE. 231 



1842, May 17.— Barom. 30-22. Haze all the day. 



Apparent Time. 

 11^ 0"^. The neutral point beneath the horizon. 



12 30 Several neutral points, three at least below the sun, as the haze files past in 

 different thicknesses. 



1842, Aug. 17.— Barom. 288. Fine day ; sky clear at 2^. 



Apparent Time. 



2h gm. Neutral point seen both below and above the sun. 



1842, Aug. 28. — A diffused haze came over a bright blue sky. 



Apparent Time. 



3h 49m_ Xhe neutral point below the sun was almost in the horizon, and Babinet's 



neutral point near the zenith. 

 5 30 Sun invisible ; mist thick ; and the polarisation everywhere positive, and 

 very feeble. 



1843, Feb. 13.--Barom. 29-7. Fine sky. 



Apparent Time. Distance of Neutral Point from Sun. 



Ih lOm 15° 0' 



1843, Feb. 16.— Barom. 29-15. Fine sky. 



Apparent Time. Distance of Neutral Point from Sun. 



12'' 57°i 11° 25' 



1843, April 30, — Barom. 3007. Not a cloud. Neutral points distinctly seen, 

 both above and below the sun. 



Apparent Time. 



4'* 15"*. Neutral point under the sun still above the horizon. 



1843, June 15. — Barom. 30-03. Splendid day ; wind east. 



Apparent Time. Distance of Neutral Point from Sun. 



121' 13m 8° 10' 



7 9 49 



1844, May 3.— Barom. 30-15. 



Apparent Time. 



ll** 3™. The neutral point below the sun distinctly seen. In order to see it well, 

 I look at it perpendicularly through a plate of glass. The bands on 

 each side of it are increased in intensity, the bands above being reversed . 



When the sky is clear the neutral point under the sun approaches to the sun 

 as his altitude increases, and coincides with the sun's centre when he is in the 

 zenith. 



As the neutral points of Arago and Babinet may be seen before the sun has 

 risen, and after he has set, they are comparatively distinct and limited in their 

 area ; but as the neutral point below the sun never can be seen unless when the 

 sui is shining, it has a less defined boundary and a wider area, owing to the 

 flood of light in which it is generally enveloped. Hence arises the great difficulty 

 of seeing it, and of detecting the form of the lines of equal polarisation which 

 surround it. For the same reason, we can hardly expect to see the secondary 

 neutral point, which must accompany it, when it rises or sets on a sea horizon in 

 a condition to produce that phenomena. 



VOL. XXIII. PART II. 3 s 



