of the Atmosphere. 291 



to the determination of the place of maximum polarization, and 

 a third to the causes which disturb the polarization of the atmo- 

 sphere, yet the chief object of his labours was to study the daily 

 variation of the polarization of the maximum point; and so 

 fully has he treated this important branch of his subject, that 

 the description of his polarimeter, of his method of using it, and 

 the discussion of his observations, with the observations them- 

 selves, occupy three-fourths of his memoir. 



In his section on the Cause of Atmospherical Polarization, 

 Dr. Rubenson is led to the same conclusion which I had de- 

 duced from my earliest observations — namely, that the light of 

 the blue sky is polarized by reflexion from the molecules of air, 

 and not from vesicles of water with parallel sides, as maintained 

 by Clausius, nor, as conjectured by others, from extremely 

 minute drops of water, nor from molecules of aqueous vapour in 

 an intermediate state between that of gas and that of vesicles. 



According to Arago, the distance of the place of maximum 

 polarization from the sun was 89° 6', the mean of six observa- 

 tions. I found 89° to be the mean of a great number of obser- 

 vations, but, like Arago, I considered 90° to be the nearest ap- 

 proximation to the place of maximum polarization. Dr. Ruben- 

 son found it to undergo, as I did, great variations, chiefly from 

 88° to 92°, the general mean of which, from his observations, 

 was 90° 2', half of which is so near to the polarizing angle of 

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

 light of the blue sky is polarized by reflexion from its particles. 



In his section on the Causes which disturb the Polarization 

 of the Atmosphere, 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 po- 

 larization was reduced by the crystals of ice floating in the atmo- 

 sphere which form the halo of 23°. 



Dr. Rubenson has not observed the secondary neutral point 

 which I found sometimes accompanying the neutral point of 

 Arago 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 f made several observations at 

 Bordeaux in order to determine the intensity of the maximum 

 polarization 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 ap- 



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

 t Comptes Rendus, vol. xxxix. p. 77 o, October 1854. 

 U2 



