CONFIRMATION OF THE UNDULATORY THECRiT. 123 



is extremely difficult to conceive any arrangement of the particles of 

 bodies by which such motions can mechanically be produced ; anc 

 this difficulty is the greater, because some fluids and some gases 

 impress a circular polarization upon light ; in which cases we cannot 

 imagine any definite arrangement of the particles, such as might form 

 the mechanism requisite for the purpose. Accordingly, it does not 

 appear that any one has been able to suggest even a plausible hypo- 

 thesis on that subject. Yet, even here, something has been done. 

 Professor Mac Cullagh, of Dublin, has discovered that by slightly 

 modifying the analytical expressions resulting from the common case 

 of the propagation of light, we may obtain other expressions which 

 would give rise to such motions as produce circular and elliptical 

 polarization. And though we cannot as yet assign the mechanical 

 interpretation of the language of analysis thus generalized, this gene- 

 ralization brings together and explains by one common numerical sup- 

 position, two distinct classes of facts ; a circumstance which, in all 

 cases, entitles an hypothesis to a very favorable consideration. 



Mr. Mac Cullagh's assumption consists in adding to the two equations 

 of motion which are expressed by means of second differentials, two 

 other terms involving third differentials in a simple and symmetrical 

 manner. In doing this, he introduces a coefficient, of which the 

 magnitude determines both the amount of rotation of the polarization 

 of a ray passing along the axis, as observed and measured by Biot, 

 and the ellipticity of the polarization of a ray which is oblique to the 

 axis, according to Mr. Airy's theory, of which ellipticity that philoso- 

 pher also had obtained certain measures. The agreement 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 pro- 

 bable, too, that the confirmation of this hypothesis involves, 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 



" Royal I. A. Trans. 1836. " Phil Trans. 1830. 



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



