of the Atmosphere, 447 



never rises till the sun is within 11° or 12° of the horizon, and 

 never sets till the sun is 11° or 12° above the horizon. 



II. On a secondary Neutral Point accompanying M. Arago's 

 Neutral Point. 



I observed the first traces of this remarkable phsenomenon 

 on the 8th of June 184-1, at 5^ 50', when the positive polari- 

 zation was strongest close to the horizon, whether land or sea, 

 and to about 1^ above it. Hence, when M. Arago's neutral 

 point rose, it did not appear Jirst in the horizon^ but about l^-^ 

 above it, the compensation being effected where the positive 

 polarization was weaker than in the horizon. When this took 

 place, we had the singular phaenomenon of a neutral point 

 xmth positive polarization on each side of it. When this phae- 

 nomenon was more fully developed, under a favourable state 

 of the horizon, the positive polarization was overcome by the 

 advancing negative polarization. The negative polarization 

 was then immediately below the ascending neutral point; but 

 at a certain distance, a few degrees below the neutral point, 

 the negative polarization was compensated by the excess of 

 positive polarization close to the horizon, and the beautiful 

 phaenomenon was seen of two neutral points separated by bands 

 of negative polarization ! This phaenomenon was best seen on 

 the sea horizon, which was marked by an obscure band a ^Q.yi 

 degrees high, that indicated the existence of a distant haze. 

 On the 21st of April 1842, I observed the secondary neutral 

 point under favourable circumstances. At 6^ 22' p.m., when the 

 primary neutral point was 15° high, the secondary one was 

 2° 50' high. At 7^ positive bands were still seen above the 

 sea line, and were strongest upon the obscure band above the 

 visible sea line. 



III. On M. Babinet's Neutral Point. 



This neutral point is situated about 18° 30' above the sun, 

 when he is rising or setting in a very clear sky. It is not so 

 easily seen as that of M. Arago, and was therefore longer in 

 being discovered. It is above the horizon during the greater 

 part of the year in great latitudes, and being above the sun, 

 it is of course always visible when the sun is above the horizon 

 in a clear sky. When the sun is in the zenith, this neutral 

 point coincides with the sun's centre. As the sun's altitude 

 diminishes, it separates from the sun's centre, its distance gra- 

 dually increasing till it becomes 18° 30', when the sun's alti- 

 tude is nothing, or at sunrise and sunset. 



The neutral point of M. Babinet must^ like that of M. 

 Arago, be accompanied, in certain states of the horizontal 



