POLARISATION OF LIGHT. 59 



a tourmaline plate, 11 and, in accordance with Newton's pre- 

 diction, of comparing the chemical composition 12 of seve- 

 ral substances 13 with their optical effects. It will be suffi- 



of polarisation the sun must be a solid or a liquid body. But 

 if on the contrary the light emanating from the sun's margin 

 give no indications of polarisation, the incandescent portion 

 of the sun must be gaseous. It is by means of such a method- 

 ical 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 re-translating the passages into French, or any other 

 of the various languages in which the Cosmos has appeared. 



11 " Sur 1'effet d'une lame de tourmaline taillee parallelement 

 aux aretes du prisme servant, lorsqu'elle est convenablement 

 situee, a eliminer en totalite les rayons reflechis par la surface 

 de la mer et meles a 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, pp. 339-343. 



12 " De la possibility de determiner 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 con- 

 tained in ammonia and in water, to carbonic acid, alcohol and 

 the diamond). See Biot et Arago, Memoir e sur les affi.nites 

 des corps pour la lumiere, Mars, 1806 ; also Memoir es mathem. 

 et phys. de llnstitut, t. vii. pp. 327-346 ; and my Memoire 

 sur les refractions astronomiques dans la zone torride, in the 

 Recueil dObserv. astron., vol. i. pp. 115 and 122. 



13 Experiences de M. Arago sur la puissance refractive des 

 corps diaphanes (de I' air sec et de lair humide) par le deplace- 

 ment des f ranges., in Moigno, Repertoire d 1 Optique mod., 1847, 

 pp. 159-162. 



