Cambridge Philosophical Society. 383 



pothesis which they have suggested to him ; of which the main 

 point is this : that the two rays in quartz are elliptically-polarized, 

 one to the right, the other to the left ; the major axes of the ellipses 

 being respectively in and perpendicular to the principal plane. Cal- 

 culations founded on this supposition represent with a very close 

 agreement, the various and complex phenomena which have 

 been noticed; and, what is more remarkable still, they not only 

 coincide in the general facts, but lead also to deviations from sym- 

 metry, such as are observed to exist in the figures. 



After the meeting, Professor Airy exhibited, 1st, A model to il- 

 lustrate Fresnel's idea, that circularly-polarized light is formed from 

 plane-polarized (when the plane of polarization is inclined 45 to 

 that of total internal reflexion), by retarding the undulations per- 

 pendicular to the plane of reflexion by one quarter of an undu- 

 lation ; and that double such a retardation shifts the plane of po- 

 larization 90; which was also shown to be the fact with Fresnel's 

 rhomb. 



2d, A new polarizing machine : the advantages of which are ; 

 that complete rings may be seen with a very small specimen : that 

 by placing the specimen in another position, the macled structure 

 may be very well seen : that circularly-polarized light may be used 

 as well as plane ; and that lamp-light may be used as well as day- 

 light. 



3d, An attempt to exhibit the coloured rings by the light of 

 heated lime ; which succeeded so far as to show the practicability 

 of this application. 



March 7. The Very Reverend the Dean of Peterborough, the 

 President, in the chair. The following presents were laid on the 

 table : A pair of the Scaup Duck (Fuligula Mania), by the Hon. 

 Richard Neville ; An egg of the Cayman, presented by Dr. Jermyn; 

 and an egg of the Great Bustard, found in Cambridgeshire, pre- 

 sented by Mr. Barren. A paper was read by R. Murphy, Esq. Fel- 

 low of Caius College, " On the general solution of equations." After 

 the meeting, the Rev. R. Willis, of Caius College, exhibited a num- 

 ber of experiments on the transverse and longitudinal vibrations of 

 strings, membranes, and solid bodies, illustrative of the recent re- 

 searches and discoveries of M. Savart. 



March 21. Dr. F. Thackeray, the Treasurer, in the chair. 

 A paper by Mr. Miller, of St. John's College, was read, " On the 

 elimination of the time from the differential equations of the motion 

 of a point, acted upon by a central force, and affected by disturb, 

 ing forces, or by the resistance of a medium." A paper, by the 

 same gentleman, was also read, containing Determinations of the 

 form and measurements of the angles of several artificial crystals ; 

 viz. sulphuret of nickel, borate of potash, nitrate of ammonia, car- 

 bazotic acid, carbazotate of potash, benzoic acid, nitrate of silver 

 and ammonia, and sulphate of copper and ammonia. The latter 

 compound appears, by comparison with the measurements of Mr. 

 Brooke, to be isomorphous or plesiomorphous with respect to 

 various other double sulphates ; viz. the sulphates of ammonia and 



magnesia, 



