446 Sir David Brewster on the Polarization 



at the part of the sky where it should be found — I at last ob- 

 served, under a very favourable state of the atmosphere, that 

 the polarization of the sky was negative in the space between 

 the risen sun and the horizon. This observation placed it 

 beyond a doubt that there must be a neutral point below the 

 sun, where that «^orflr^/t;e polarization passed mio positive pola- 

 rization; and by concealing the sun from view, and admitting 

 no light to the eye but what came from the probable place of the 

 neutral point, 1 succeeded in discovering it. After commu- 

 nicating this discovery to M. Babinet*, early in 1845, he made 

 several ineffectual attempts to confirm it; and it was not till 

 the 23rd of July IS^G, when the state of (he sky was peculiarly 

 favourable for the observation, that he succeeded in obtaining 

 a distinct view of itf. 



Before proceeding to explain the map of the lines of equal 

 polarization in the pure blue sky, I shall give a brief account 

 of my observations on the three neutral points to which 1 

 have referred : — 



I. On M. Arago's Neutral Point. 



In the normal state of the lines of equal polarization, 

 namely, when the sun is in the horizon, this neutral point 

 is about 18^° above the horizon or above the antisolar point; 

 but when the sun is about 11° or 12° above the horizon, 

 and the antisolar point of course as much below it, the neutral 

 point is in the horizon, and consequently only 11° or 12° 

 above the antisolar point. As the sun descends to the horizon, 

 and the antisolar point rises, the distance of the neutral point 

 from the latter gradually increases ; and when the sun reaches 

 the horizon, the neutral point is 16^° above it, and therefore 

 18|° distant from the antisolar point. After the sun has 

 set, the distance of the neutral point from the antisolar 

 point increases ; that is, it rises faster than the sun descends, 

 and its maximum distance when the twilight is very faint, is 

 about 25°. 



In the latitude of St. Andrews, M. Arago's neutral point is 

 above the horizon all the day between the middle of November 

 and the end of January. In the other months of the year it 



* Comptes Rendus des Seances de t'Acad. des Scieiices, torn. xxii. p. 801- 

 803,1845,17th Mars. 



•f- Comptes Rendus, &c,,i\n\\tt'^, 1846, torn, xxiii. p. 195; andAoiitS, 

 1846, torn, xxiii. p. 233. " M. Brewster," says M. Babinet, "a sans doute ^te 

 guide dans sa recherche par des vues theoriques ; autrement il me parait 

 pen probable qu'il eut fait, par observation seul de la polarization atmo- 

 spherique, la decouverte remarquable de ce point neutre si difficile a recon- 

 naitre, et que, depuis luij' avals plusieur fois tente inutilement de retrouver." 

 Ibid. p. 235. 



