( 247 ) 



XXI. — Additional Observations on the Polarisation of the Atmosphere, made at 

 St Andrews in 1841, 1842, 1843, 1844, and 1845. By Sir David Brewster, 

 K.H., D.C.L., F.R.S., &c. 



(Read 2d January 1866.) 



Since the publication of my " Observations on the Polarisation of the Atmo- 

 sphere," a long and elaborate Memoir on the same subject, by Dr 11. Rubenson, 

 has appeared in the Acts of the Royal Society of Sciences of Upsal.* The obser- 

 vations which it contains were made with the finest instruments, and with a 

 degree of accuracy which had not been attempted by previous observers. They 

 were begun at Upsal in 1859, and carried on at Rome between the 6th of June 

 and the 5th of August 1861, at Segni in the Campagna, between the 6th and the 

 27th of August 1861, and at Rome from the 5th of October 1861 to the 27th of 

 July 1862. 



Although Dr Rubenson has devoted a section of his work to ascertain the cause 

 of atmospherical polarisation, another section to the determination of the place of 

 maximum polarisation, and a third to the causes which disturb the polarisation 

 of the atmosphere, yet the chief object of his labours was to study the daily varia- 

 tion of the polarisation 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 

 themselves, occupy three-fourths of his Memoir. 



In his section on the Cause of Atmospherical Polarisation, Dr Rubenson is led 

 to the same conclusion which I had deduced from my earliest observations, 

 namely, that the light of the blue sky is polarised 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 polarisation from 

 the sun 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 considered 90° to be the nearest 

 approximation to the place of maximum polarisation. Dr Rubenson found it to 

 undergo, as I did, great variations, chiefly from 88° to 92°, the general mean of 



* Series 3d, torn. v. This Memoir has been published as a separate work in 4to, pp. 238. 

 Upsal, 1864. 



VOL. XXIV. PART II. 3 Y 



