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VII. On the Polarization of Light by Air mixed with Aqueous 

 Vapour. By Professor Haidinger*. 



To Professor Tyndall, F.R.S. 



Dormbach near Vienna, 

 My dear Sir, June 13, 1869. 



YOUR late experiments and reports on the polarization of 

 light by cloudy matter (Proceedings of the Royal Society, 

 No. 108, vol. xiii. pp. 223 &c, Jan. 14, 1869) have made a deep 

 impression on my mind. 



Permit me to advert to an ancient observation of mine relating 

 to a subject of the kind, but under circumstances widely different, 

 which nevertheless I now very much should wish you may think 

 worthy of a glance in the development of your further inquiries. 



I have observed the polarization of light by air mixed with 

 watery vapour. I gave an account of it in PoggendorfFs An- 

 nalen for 1846, vol. lxviii. pp. 73-87 (77). Abbe Moigno, like- 

 wise, from Poggendorff, gave a report of it in the fourth volume 

 of his Repertoire d'Optique Moderne, 1850, pp. 1338 & 1339. 

 Both were accompanied with diagrams. In the vapour-bath, of 

 course, 1 had no optical apparatus with me; but having shortly 

 before been struck with the appearance of the brushes of polarized 

 light, or of polarization (Polarisations-buschel) , I was well pre- 

 pared to test or to recognize polarized light under certain circum- 

 stances with the naked eye, by trying whether I could not see 

 these brushes. 



It is perhaps hardly discreet of me to demand you should be 

 at the trouble of searching out old volumes ; so I beg you will 

 permit me just to translate that portion of one of my old papers 

 which refers to the subject. 



" Brushes of polarization observed in watery vapour. 



" White bows or nebulous arches (Nebelbogen) have been ob- 

 served in fogs or mists, having nearly the apparent diameter of 

 rainbows. The light of the rainbow has been found to be pola- 

 rized by Biot and Sir David Brewster, conformably to the well- 

 known explanation by single reflection of the light of the sun for 

 the interior rainbow, and by double reflection for the exterior 

 rainbow. 



" I had an opportunity to observe the white vapour-bows or 

 arches in the vapour-baths of the ' Sorbienbad/ a most merito- 

 rious establishment, conducted by M. Marawetz in the suburb 

 Landstrasse in Vienna. Since my observation a new building 

 has been raised on the east side, so that it is no longer possible 

 there to repeat the observation. 



"The sun shone bright at 7 o'clock in the morning, under a 

 * Communicated bv Professor Tvndall. 



