248 SIR DAVID BREWSTER ON THE 



which, from his observations, was 90° 2', half of which is so near to the polarising 

 angle of air, which is 45° 0' 32", as to place it beyond a doubt that the light of 

 the blue sky is polarised by reflexion from its particles. 



In his section on the Causes which Disturb the Polarisation of the Atmo- 

 sphere, Dr Rubenson found, as I did, that clouds and fogs and smoke were the 

 cause of the greatest perturbations ; and he also found, as I had done,* that the 

 intensity of the polarisation was reduced by the crystals of ice floating in the 

 atmosphere which form the halo of 23°. 



Dr Rubenson has not observed the secondary neutral point which I found 

 sometimes accompanying the neutral point of Akago, when it rises above the 

 horizon, or is setting beneath it, and he has never been able to see, even under 

 the fine sky of Italy, the neutral point which I discovered under the sun, and 

 which, I believe, has not been seen by any other observer than M. Babinet. 



In 1854, M. Felix Bernard! made several observations at Bordeaux, in order 

 to determine the intensity of the maximum polarisation at different hours of the 

 day. Though made only on four days of the month of October (from the 16th to 

 the 19th inclusive), he found " that in proportion as the sun approaches the 

 meridian the value of the maximum polarisation diminishes ; that this value in- 

 creases, on the contrary, in a continuous manner as the sun recedes from the 

 meridian, and that it reaches its maximum when the sun is very near the 

 horizon, the amplitude of this variation being about 009." 



On the 16th October 1854, the maximum polarisation increased gradually after 

 mid-day from 25° to 0° of the sun's altitude, from 06236 to 0-7051 ; and on the 

 19th October, from 5° to 35° of the sun's altitude, it diminished from 0-7083 to 

 06106. On these two days the maximum polarisation, at an altitude of 20°, was 

 0-6582, and 0-6464 respectively, the mean of which is 0-6523, differing only 0-12 

 from 0*64, as computed from Fresnel's formula by M. Bernard, from my obser- 

 vation in 1842, that when the sun's altitude was 20°, the intensity of the maxi- 

 mum polarisation at 90° from the sun was equivalent to that which would be 

 produced by reflexion from the surface of glass, whose index of refraction was 

 1-486, at an angle of 6b° 30' J. 



Before v he became acquainted with the Memoir of M. Bernard, Dr Rtjbenson 

 had completed his observations on the same subject ; and, though they lead to a 

 similar result, yet they possess a peculiar value from their having been made with 

 the finest instruments, in different localities, — in almost all the seasons of the 

 3 T ear, and under various states of the atmosphere. 



From a careful examination of his observations, Dr Rubenson arrives at the 



* Treatise on Optics, p. 394, and Edin. Trans., vol. xxiii. p. 226. 

 f Comptes Eendus, torn, xxxix. p. 775, October 1854. 



+ Johnston's Physical Atlas — Meteorology, p. 10; or Phil. Mag., series 3d, vol. xxiv. p. 453, 

 December 1847- 



