TRANSACTIONS OP THE SECTIONS. 7 



On an attempt lately made by M. Laurent, to explain on meclmnical principles 



the Phcenomenon of Cirmlar Polarization in Liquids. 



By Professor MacCullagh. 



The author showed that this attempt had not succeeded. M. Laurent supposes 

 the particles of the luminiferous ifither not to be simply material points, but to have 

 dimensions which are not insensible when compared with their distances ; and on this 

 hypothesis he deduces a system of differential equations, the integrals of which he 

 conceives to represent the phaenomenon in question. The integrals given by M. Lau- 

 rent are, however, altogether erroneous, though this circumstance was not noticed 

 by M. Cauchy in the remarks and comments which he made on M. Laurent's me- 

 moir. The true integrals of these equations (supposing the equations to be correctly 

 deduced) were shown by Professor MacCullagh to indicate motions of the aether 

 which do not correspond to the observed pheenomena. The account of M. Laurent's 

 theory, with M. Cauchy's remarks upon it, will be found in the eighteenth volume 

 of the ' Coraptes Rendus' of the Academy of Sciences of Paris. 



On certain points connected with Elliptic Polarization of Light. 

 By the Rev. Professor Powell, M.A., FM.S. 



The peculiar property impressed upon light reflected from metal, and previously 

 polarized at 45° to the plane of reflexion, discovered by Sir David Brewster in 1830, 

 and named by him elliptic polarization, was examined by him chiefly with regard to 

 the effects produced by a second reflexion from the same metal, — when the plane 

 polarization is restored, but with its plane changed by a certain angle, which at the 

 maximum characterizes each metal. From this, however, we cannot infer what pre- 

 cise effect is produced by the^rs^ reflexion alone. It also appears that the ellipticity 

 is small or insensible at small incidences, — arrives at a maximum for most metals at 

 between 70° and 80°, and then decreases again, up to 90°. 



The author of this communication has examined some properties of light of this 

 kind by means of the changes in the polarized rings, after one reflexion at different 

 incidences. 



In all degrees of ellipticity the rings have branches more or less faint correspond- 

 ing to the degree of dislocation, in all relative positions of the planes of polarization 

 and analysation. At small incidences they are dark and bright in the rectangular 

 positions. 



The position of the darkest branches, with respect to the plane of reflexion, changes 

 at different incidences, in a manner somewhat analogous to what takes place in the 

 reflexion from glass, though it is not at all expressed by Fresnel's law. 



At the smallest incidences the position is always different from 45° ; being deter- 

 mined by an arc considerably greater than 45°, as measured from the plane of 

 reflexion. 



At greater incidences the arc diminishes ; and at the maximum the position coin- 

 cides with the plane of reflexion. The first-named arc varies with different metals ; 

 but the last result is common to all. The intermediate change is more or less gra- 

 dual in different cases. 



The author is engaged in measuring these arcs for a series of metals, but he is not 

 able at present to trace any relation between them and those determined by Sir David 

 Brewster after two reflexions. 



In the author's paper in the Phil. Trans., 1842, a formula is given for elliptically 

 polarized rings with different retardations : this formula being somewhat generalized, 

 includes an expression for a change of plane ; and explains some portion of the phee- 

 nomena which has not been precisely discussed, especially the peculiar appearance of 

 the rings when the plane of analysation is at 45° to that of polarization. 



It does not appear that any theoretical connexion has been yet made out between 

 this virtual change of plane and the retardation which changes with the incidence. 



The author is anxious to call attention to this subject in the hope of eliciting from 

 those members who have examined it some resiilts which may enable us to compare 

 theory and observation. 



