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XXh— 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., r.R.S., &c. (Plate XII.) 



(Read 16tli March 1863.) 



During the last half century, observations on the polarisation of the atmo- 

 sphere vrere made by several eminent observers — by Arago, Delezenne, Babinet, 

 and Zantedeschi ; but no result of special importance v^^as obtained till Arago 

 made the great discovery that there existed in the atmosphere a point, or spot, 

 in which there is no polarisation. At sunrise or sunset he found that this neutral 

 point was 20° or 30° above the point opposite to the sun, or what we may call 

 the Antisolar point. The name of Aragds neutral point has been given to this 

 spot without polarisation. It is best seen after sunset. At St Andrews it is 

 above the horizon all the day, between the middle of November and the end of 

 January. 



In the year 1840, M. Babinet made the next important discovery respecting 

 the polarisation of the atmosphere. When on a visit to the sea-coast, he dis- 

 covered that there was a neutral point as far above the sun as Arago's neutral 

 point was above the antisolar point.* To this spot the name of Babinefs neutral 

 point has been given. It is most distinctly seen immediately after sunset, but is 

 much fainter than the other, on account of the discoloration of the sky by the 

 yellow light of the setting sun. 



Upon hearing of this discovery, I saw that we had now the elements for 

 determining the laws of the polarisation of the atmosphere ; and, being ambi- 

 tious of succeeding in such an inquiry, I devoted four years to the study of the 

 subject. 



My observations commenced on the 28th April 1841 ; and I made many 

 hundred on the position of the two neutral points, — on their change of place 

 under different states of the weather, different degrees of transparency in the 

 atmosphere, different degrees of light in the sky, and different altitudes of the 

 sun. I measured also the maximum polarisation of the atmosphere in different 

 azimuths between that which passed through the sun and the zenith, and that 

 which, at sunset, passed through the sun and the horizon. These observations 



* Comptes Kendus, &c., 1840. Tom. xi. p. 618. 

 VOL. XXTII. PART II. 3 N 



