of the Atmospliere. 167 



take the precaution of shielding the head and the polariscope 

 from the direct rays of the sun, and the reflexion from the earth. 



M. Brewster was doubtless guided in his research by 



theoretical views ; otherwise it appears to me very improbable 

 that, by merely observing the atmospherical polarization, he 

 could have made the remarkable discovery of this neutral point, 

 so difficult to see, and which, after him, I have several times 

 tried in vain to rediscover The small quantity of polar- 

 ized iight which is observed between the neutral point of Brew- 

 ster and the sun, seems to me to reach the limit which it is pos- 

 sible to observe, and perhaps to exceed the limit which it is pos- 

 sible to measure"*. 



1841, November 17. — Barom. 29*43 in. 



Apparent time, 

 h m 



12 0. The polarization between the sun and the horizon deci- 

 dedly negative, but no neutral point there. 



1842, February 16.— Barom. 30*16 in. 



12 0. Sun's altitude about 21° 13'; a China-ink muddy sky. 

 There is clearly a neutral space below the sun and a 

 little above the horizon. The bands on the sun and 

 below him are negative ; but as the negative polariza- 

 tion becomes very weak, it must pass into a neutral point. 



1842, February 18. — Cold, wind west, and rather stormy. 



12 The neutral point below the sun was distinctly seen, the 

 polarized bands being negative over and below the 

 sun, and below the neutral point positive down to the 

 horizon. The sun's altitude was about 22°, and the dis- 

 tance of the neutral point from the sun about 15°. 



1842, February 21. — Barom. 20*44 in. Fine, dry day; wind 

 west. 



12 39 Neutral point 6|° above the horizon, audits distance from 

 the sun 15° 0'. 

 The positive polarization between the neutral point and 

 the horizon was compensated by the refraction of one 

 plate of glass, at 20° of incidence. The neutral line was 

 convex towards the sun in the west horizon. 



1842, March 10. — Sky clear in zenith ; wind west. 



11 15 The neutral point distinctly seen below the sun. 

 1842, April 3. — Barom. 29*8 in. Fine, clear sky; cold; hail 

 in the afternoon. 



1145 Neutral point below the sun very distinctly seen, the sun's 

 altitude being about 39^°, and the height of the neutral 

 point 2(S\°. The distance of the neutral point from the 

 sun was 13°. The bands over the sun down to the 

 neutral point were negative, and those below it down to 

 the horizon positive. 



* Comptes Rendus, &c. 1846, vol. xxiii. p. 234. 



