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XV. Observations on the Polarization 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* 



[With a Plate.] 



DURING the last half century observations on the polariza- 

 tion of the atmosphere were made by. several eminent 

 observers — by Arago, Delezenne, Babinet, and Zantedeschi ; but 

 no result of special importance was obtained till Arago made the 

 great discovery that there existed in the atmosphere a point or 

 spot in which there is no polarization. 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 Arago's neutral point has been given to this spot 

 without polarization. It is best seen after sunset. At St. An- 

 drews 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 disco- 

 very respecting the polarization of the atmosphere. When on a 

 visit to the sea-coast, he discovered that there was a neutral point 

 as far above the sun as Arago's neutral point was above the 

 antisolar point f. To this spot the name of Babinet' s 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 polarization of the 

 atmosphere ; and being ambitious of succeeding in such an in- 

 quiry, I devoted four years to the study of the subject. 



My observations commenced on the 28th of 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 polarization 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 were not difficult to 

 make when the sky was clear and blue; but in studying the 

 part of the atmosphere between the sun and the horizon, I was 

 perplexed beyond measure with the feeble and uncertain indica- 



* From the Transactions of the Royal Society of Edinburgh, vol. xxiii. 

 part 2. Communicated by the Author, 

 t Comptes Rendus, &c, 1840, vol. xi. p. 618. 



