228 



SIR DAVID BREWSTER 



Observations on the Neutral Point below the Sun. 



This neutral point, which I discovered in 1842, is much more diflficult to bej 

 seen than that of Babinet. In November, December, and January, it cannot be | 

 seen in our latitude, unless when, early in November and late in January, a high 

 degree of polarisation in the sky brings it above the horizon at noon. 



The following interesting remarks of M. Babinet on his successful attempt to j 

 confirm the existence of this neutral point, explain in the clearest manner the 

 causes of the difficulties which were experienced, and which every future observer 

 will experience in observing this remarkable spot, with its surrounding polarisa- 

 tions : — 



" On the 23d July," says M. Babinet, " after having observed from half-hour 

 to half-hour the polarisation of the sky below the sun, the regularity of this 

 polarisation appeared to change after 4^ ; and from 4|'' to 5^^ I observed in 

 placing the bands horizontally — 



" \st^ A space without polarisation below the sun. 



" 2d, Below this space a second space, when the bands were certainly seen ; 

 and, 



" 3<?, Lower still a neutral space, where no bands were seen ; and, 



" 4^A, In approaching to the horizon, a fourth space, where the bands were 

 very visible. The phenomenon is therefore no longer doubtful ; but the immense 

 l)rightness of the sun in a clear day, the intense illumination of the atmosphere 

 in the region immediately below him, and the reflexion from the strongly illumi- 

 nated earth, all concurred in rendering this observation difficult to make, and 

 very painful to the eyes, even if we 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 polarisation, 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 polarised light which 



is observed between the neutral point of Brewster and the Sun, seems to me to 

 reach the limit which it is possible to observe, and perhaps to exceed the limit 

 which it is possible to measm-e." * 



1841, Nov. 17.— Barom. 29-43. 



^\pparent Time. 

 12^ 0™. The polarisation between the sun and the horizon decidedly nejraitve, hut no 

 neutral point there. 



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



