180 On the Polarization of the Atmosphere. 



5th of August, 1861 ; at Segni* between the 5th and the 27th 

 of August, 1861 ; and at Rome from the 5th of October, 1861, 

 to the 27th of July, 1862, with the finest instruments and with 

 a degree of accuracy which had not been attempted by previous 

 observers. In his elaborate work, occupying 238 pagesf, he has 

 treated the subject in twelve sections, with copious tables of his 

 observations, and illustrated with four plates. 



In his first section, on the history and literature of the subject, 

 he has done ample justice to his predecessors in this inquiry. 



In his section on the Cause of Atmospherical Polarization, 

 Dr. Rubenson is led to the same conclusion as that contained 

 in the preceding pages, namely, that the light is polarized by 

 reflexion from the particles of air, and not from vesicles of water 

 with parallel faces, as supposed by Clausius, nor, as others have 

 supposed, from extremely small and transparent drops of water, 

 nor from molecules of aqueous vapour in an intermediate state 

 between that of gas and that of vesicles. 



The place of maximum polarization, according to Arago, was 

 89° 6', the mean of six observations. I found 89° to be the 

 mean of a great number of observations, but, like Arago, I con- 

 sider 90° to be the nearest approximation to the place of maximum 

 polarization. Dr. Rubenson found it to undergo, as I did, great 

 variations, chiefly from 88° to 92°, the general mean of which 

 was 90° 2'. 



In observing the daily 'variation of the polarization of the 

 maximum point, Dr. Rubenson found that it was subject to a 

 diminution during the morning, and an augmentation during the 

 evening, without being able to assign with certainty the precise 

 hour of minimum polarization — a result which doubtless arises 

 from the greater quantity of aqueous vapour in the air in the 

 morning than in the evening. 



In treating of the causes which disturb the polarization of the 

 atmosphere, Dr. Rubenson found, as I did, that clouds and fogs 

 and smoke were the most important. He found also, as I had 

 done J, that the halo formed by crystals of ice reduces the inten- 

 sity of polarization. 



Dr. Rubenson has not observed the secondary neutral point 

 which I found to accompany sometimes that of Arago. He once 

 observed, however, a neutral point accompanying that of Rabinet, 

 but arising apparently from a different cause from that which pro- 

 duces the secondary neutral point of Arago. 



Dr. Rubenson has never been able to see, even under the fine 



* Long. 52 m East, and lat. 41° 41 f *l. 



t Memoire sur la polarisation de la lumiere atmospherique. 4to. Upsal, 

 1864. Extracted from the 'Acts of the Royal Society of Sciences of Upsal,'' 

 vol. v. 



X Treatise on Optics, p. 394. 



