ON THE POLARISATION OF THE ATMOSPHERE. 



227 



a quarter of a mile distant invisible. The sun shone occasionally, showing his 

 pale white disc. 



Apparent Time. 

 10'' 20" A.M. 



2** 55"^. 



The bands were negative from the zenith through the sun to the horizon, 

 and positive to the opposite horizon. The maximum polarisation 

 was 90° from the sun, but was so weak that it was compensated by 

 the refraction of a plate of ground glass, at an angle of incidence of 

 about 30°, the negative bands being then scarcely visible. 

 The vapour still rising copiously, and the place of the sun not visible. The 

 bands were hardly visible, the polarisation, when a maximum, being 

 compensated by the refraction of a plate of glass, at an angle of 5°. 



1843, May 3.— Barom. 29-65. Easterly haur. 



Apparent Time. 



^h 25°i. Mist flying before the sun, and the neutral point oscillating from near the 

 sun to the zenith as the mist thickens. 



1843, June 21. — Barom. 2975. Fine day ; wind west. 



Apparent Time. 

 7h 46m 



33 

 14 



R=Maximum Polarisation. 

 In Zenith. In Horizon. 



29° 

 30 



1844, Feb. 3.— Snow covering the ground. 



^ _,. R=;Maximum Polarisation. 



Apparent Time. ^^ Zenith. In Horizon. 



4I1 llm 26° 23° 



1844, April 15. 



Apparent Time. 

 5h 54m 

 6 50 



Height of Neutral Point ahove 

 the Antisolar Point. 



15° 55' 

 17 57 

 16 28 



Height of Neutral Point above 

 the Antisolar Point. 



17° 55' 



Height of Neutral Point above 

 the Antisolar Point. 



25° 6' 

 18 10 



1844, June 10.— Barom. 2970. 



_. R^^Maximum Polarisation. Height of Neutral Point above 



Apparent lime. ^^ Zenith. In Horizon. the Antisolar Point. 



8'' 201° 24f ... 18° 14' 



In a normal state of the sky, when the sun is rising or setting in a fine day 

 without clouds, the neutral point of Babinet is situated about 18° 30' above the 

 sun. Owing to the great quantity of light in the neighbourhood of the sun, this 

 neutral point is not so easily seen as that of Arago, and escaped the scrutiny of 

 that distinguished observer. In high latitudes it is above the horizon the greater 

 part of the year, and, being above the sun, it is of course always visible in a clear 

 sky, when he is above the horizon. When the sun is in the zenith, this neutral point 

 coincides with the sun's centre, its distance from the sun gradually increasing 

 till it becomes 18° 30' at sunrise or sunset, when the sun's altitude is nothing. 



Like that of Aeago, the neutral point of Babinet must be accompanied, in 

 certain states of the horizontal sky, with a secondary neutral point, but I have 

 never had an opportunity of observing it. 



VOL, XXIII. PART II. 3 R 



