On the Polarization of the Atmosphere, 119 



tions of the polariscope. The sky between the sun and the 

 horizon is always the most impure portion of it; and the flood 

 of light streaming from the sun unfits the eye for detecting faint 

 traces of colour. Theoretical considerations had led me to 

 believe that a neutral point should be found between the sun 

 and the horizon, and certain indications of the polariscope, at 

 places around its probable locality, entitled me to infer that it 

 did exist; but as an inference was not satisfactory, I watched 

 every favourable state of the sky in the hope of obtaining a more 

 direct result. 



An opportunity of doing this at last presented itself to me on 

 the 28th of February, 1842, when the sun was in the meridian 

 with an altitude of 22°. The spot beneath the sun was fortu- 

 nately visible from the end of a long dark passage running- 

 north and south, and having concealed the sun himself, and 

 every part around him except the probable position of the spot, 

 I obtained a most distinct view of the new neutral point, situated 

 about 15° or 16° below the sun. After communicating this 

 discovery to M. Babinet early in 1845, he made several ineffec- 

 tual attempts to confirm it, and it was not till the 23rd of July, 

 1846, that he succeeded in obtaining a distinct view of this neu- 

 tral point. Following the usual practice, the French have given 

 to this spot the name of Brewster's neutral point (p. 128). 



Fio-. l. 



MAX. POLARIZATION. 



BABINETS 



N. P. 



ARAGO'S N. P. • ^^ 1 BREWSTER'S N. P. 



H 



ANTISOLAR 

 POINT 



HORIZON. 



With these three elements of atmospherical polarization, 

 shown in fig. 1, I shall now proceed to determine their position 

 in relation to the sun and the antisolar point, and ascertain the 

 changes which they undergo from variations in the optical and 

 meteorological state of the atmosphere. 



Observations on Arago's Neutral Point, 



In observing the different neutral points, I employed chiefly 

 Savart's band polariscope, which was kindly presented to me by 



