27 



Professor Mac Cullagh made a verbal communication on 

 the probable nature of the light transmitted by the diamond 

 and by gold leaf. He conceives that as there is a change 

 of phase caused by reflexion from these bodies, so there is 

 also a change of phase produced by refraction; the change 

 being different according as the incident light is polarized in 

 the plane of incidence, or in the perpendicular plane. Con- 

 sequently, if the incident ray be polarized in any interme- 

 diate plane, the refracted ray should be elliptically polarized ; 

 and on examining the light transmitted by gold leaf, this 

 was found to be the case. Of course the same thing is true 

 of the light which enters the other metals, and which is sub- 

 sequently absorbed. The same remark explains the appear- 

 ance of double refraction in specimens of the diamond which 

 give only a single image ; and it is hkely that other precious 

 stones will be found to possess similar properties. Mr. 

 Mac Cullagh has obtained a general formula for the difference 

 of phase between the two component portions of the refract- 

 ed light — one polarized in the plane of incidence, and the 

 other perpendicular to it. He finds from this formula, that 

 the difference of phase, which is nothing at a perpendicu- 

 lar incidence, increases until it becomes equal to the charac- 

 teristic at an incidence of 90° ; and when the light emerges 

 into air, the difference of phase is doubled. The formula 

 has not yet been submitted to the test of experiment. 



Mr. Mac Cullagh then read a paper " on the Laws of 

 CrystalHne Reflexion and Refraction." 



In this paper the solution of the following problem is 

 given for the first time : — 



Supposing a ray of light, polarised in a given plane, to fall 

 on a doubly refracting crystal, it is required to find the plane 

 of polarisation of the reflected ray, and the proportion be- 

 tween the amplitudes ofmbration in the incident, the reflect- 

 ed, and the two refracted rays. 



