488 HISTORY OF OPTICS. 



between the two sets of measures 12 thus brought 

 into connexion is such as very strikingly to confirm 

 Mr. Mac Cullagh's hypothesis. It appears probable, 

 too, that the confirmation of this hypothesis in- 

 volves, although in an obscure and oracular form, a 

 confirmation of the undulatory theory, which is the 

 starting-point of this curious speculation. 



5. Elliptical Polarization of Metals. The effect 

 of metals upon the light which they reflect, was 

 known from the first to be different from that which 

 transparent bodies produce. Sir David Brewster, 

 who has recently examined this subject very fully 13 , 

 has described the modification thus produced, as 

 elliptic polarization. In employing this term, " he 

 seems to have been led," it has been observed 14 , "by 

 a desire to avoid as much as possible all reference 

 to theory. The laws which he has obtained, how- 

 ever, belong to elliptically-polarized light in the 

 sense in which the term was introduced by Fres- 

 nel." And the identity of the light produced by 

 metallic reflection with the elliptically-polarized 

 light of the wave theory, is placed beyond all doubt, 

 by an observation of Professor Airy, that the rings 

 of uniaxal crystals, produced by Fresnel's elliptic- 

 ally-polarized light, are exactly the same as those 

 produced by Brewster's metallic light. 



6. Newton's Rings by Polarized Light. Other 

 modifications of the phenomena of thin plates by 



12 Royal I. A. Trans. 1836. 13 Phil Trans. 1830. 



'* Lloyd, Report on Optics, p. 372. (Brit. Assoc.) 



