166 Sir David Brewster on the Polarization 



In a normal state of the sky, when the sun is rising or setting 

 in a fine day without clouds, the neutral point of Babinet is 

 situated about 18° 30' above the sun. Owing to the great 

 quantity of light in the neighbourhood of the sun, this neutral 

 point is not so easily seen as that of Arago, and escaped the 

 scrutiny of that distinguished observer. In high latitudes it is 

 above the horizon the greater part of the year, and, being above 

 the sun, it is of course always visible in a clear sky, when he is 

 above the horizon. When the sun is in the zenith this neutral 

 point coincides with the sun's centre — its distance from the sun 

 gradually increasing till it becomes 18° 30' at sunrise or sunset, 

 when the sun's altitude is nothing. 



Like that of Arago, the neutral point of Babinet must be 

 accompanied, in certain states of the horizontal sky, with a 

 secondary neutral point, but I have never had an opportunity of 

 observing it. 



Observations on Brewster's Neutral Point. 



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

 difficult to be seen than that of Babinet. In November, Decem- 

 ber, and January it cannot be seen in our latitude, unless when, 

 early in November and late in January, a high degree of polari- 

 zation in the sky brings it above the horizon at noon. 



The following interesting remarks of M. Babinet on his suc- 

 cessful attempt to confirm the existence of this neutral point, 

 explain in the clearest manner the causes of the difficulties which 

 he experienced (and which every future observer will experi- 

 ence) in observing this remarkable spot with its surrounding 

 polarizations : — 



"On the 23rd July," says M. Babinet, "after having ob- 

 served from half-hour to half-hour the polarization of the sky 

 below the sun, the regularity of this polarization appeared to 

 change after 4 h ; and from 4| h to 5^ h I observed, in placing the 

 bands horizontally — 



" 1st. A space without polarization below the sun ; 



"2nd. Below this space a second space, where the bands were 

 certainly seen. 



" 3rd. Lower still a neutral space, where no bands were seen ; 

 and, 



" 4th. In approaching to the horizon, a fourth space, where 

 the bands were very visible. The phenomenon is therefore no 

 longer doubtful; but the immense brightness 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 illuminated earth, all concur in rendering this obser- 

 vation difficult to make,. and very painful to the eyes, even if we 



