POLARIZATION OF LIGHT. 4? 



mosplieric strata, and even the smallest amount of water they 

 contain, of scrutmizing the depths of the ocean and its rocka 

 by means of a tourmaline plate,=^ and, in accordance with 

 Newton's prediction, of comparing the chemical compositionf 

 of several substances^ with their optical effects. It will be 

 sufficient to mention the names of Aiiy, Arago, Biot, Brew- 

 ster, Cauchy, Faraday, Fresnel, John Herschel, Lloyd, Ma- 

 lus, Neumann, Plateau, Seebeck, to remind the sci- 

 entific reader of a succession of splendid discoveries and of 

 their happy applications. The great anfi intellectual labors 

 of Thomas Yomig more than prepared the way for these im- 

 portant efforts. Arago's polariscope and the observation of 

 the position of colored fringes of diffraction (in consequenc*' 

 of interference) have been extensively employed in the pros 

 ecution of scientific inquiry. Meteorology has made equa.' 

 advances with j:)/i?/sz'caZ aUronomij in this new path. 



However diversified the power of vision may be iu differ 

 ent persons, there is nevertheless a certain average of orgai? 



is by means of such a melhodical sequence of observations that we njs*: 

 acquire exact ideas regarding the physical constitution of the sun.^ 

 (On the Envelopes of the Sun, see Arago, in tlie Annuaire pour 1846 

 p. 464.) I give all the circumstantial optical disquisitions which I havr 

 borrowed from the manuscript or printed woi'ks of my friend, in his 

 own words, in order to avoid the misconceptions to which the variations 

 of scientific terminology might give rise in retranslating the passages 

 into French, or any other of the various languages in which the Cosmos 

 has appeared. 



* " Sur I'efFet d'uue lame de tourmaline taillce parallelement aux 

 aretes du prisme servant, lorsqu'elle est convenablement situee, a eli- 

 miner en totalite les rayons reflecliis par la surface de la mer et meles k 

 la lumiere provenant de I'ccaeil." *' On the effect of a tourmaline plate 

 cut parallel to the edges of the prism, in concentrating (when placed in 

 a suitable position) all the rays of light reflected by the surface of the 

 sea, and blended with the light emanating from the sunken rocks." 

 See Arago, Listntctions de la Bonite, in the Annuaire pour 1836, p. 339 



343. . , . " 



t " De la possibilite de determiner les pouvoirs refringents des corps 

 d'aprcs leur composition chimique." On the possibility of determining 

 the refracting powers of bodies according to their chemical composition 

 applied to the ratio of the oxygen to the nitrogen in atmospheric an*, 

 to the quantity of hydrogen contained in ammonia and in water, to car- 

 bonic acid, alcohol, and the diamond). See Biot et Arago, Mimoire 

 grtr les Affinites des Corps poxir la Lumiere, JMars, 1806; also Mimoircs 

 Mathem. et Phys. de V Inst Hut, t. vii., p. 327-346 ; and my Mimoire gur 

 les Refractions Astronomiqtics dans la Zone Torride, in the RecueH 

 d'Observ. Astron., vol. i., p. 115 and 122. 



t Experiences de M. Arago sur la puissance Refractive des Corps 2>»- 

 ephanes {de Vair sec et de Vair humide) par le De placement des Franges, 

 ■ in Moi^iio, Repertoire d'Optique Mod., 1847, p. 159-162. 



