POLARIZATION OF LIGHT. 47 



mospheric strata, and even the smallest amount of water they 

 contain, of scrutinizing the depths of the ocean and its rocks 

 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 Airy, 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 and intellectual labors 

 of Thomas Young 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 consequence 

 of interference) have been extensively employed in the pros- 

 ecution of scientific inquiry. Meteorology has made equal 

 advances with physical astronomy in this new path. 



However diversified the power of vision may be in differ- 

 ent persons, there is nevertheless a certain average of organ- 

 is by means of such a methodical sequence of observations that we may 

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

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

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

 borrowed from the manuscript or printed works 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 1'effet d'une lame de tourmaline taillee parallelement aux 

 aretes du prisme servant, lorsqu'elle est convenablement situee, a 61i- 

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

 la lumiere provenant de 1'ecueil." " 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, Instructions de la Bonite, in the Annuaire pour 1836, p. 339 



343. 



t " De la possibilit6 de d6terminer les pouvoirs refringents des corps 

 d'apres 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 air, 

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

 bonic acid, alcohol, and the diamond). See Biot ct Arago, Mtmoire 

 sur les Ajfinites des Corps pour la Lumiere, Mars, 1806; also Mlmoirts 

 Mathem. et Phys. de V Inslilut, t. vii., p. 327-346 ; and my M6moire gur 

 les Refractions Astronomiques dans la Zone Torride, in the Recueit 

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



t Experiences de M. Arago sur la puissance Refractive des Corps D\- 

 aphanes (de I' air sec ct de I' air humide) par le Replacement des Franges, 

 iu Moigno, Repertoire d'Oplique Mod., 1847, p. 159-K52. 



