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 likely 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 
Crystalline Reflexion and Refraction.” 
In this paper the solution of the following problem is 
given for the first time :— 
Supposing aray of light, polarized in a given plane, to fall 
on a doubly refracting crystal, it is required to find the plane 
of polarization of the reflected ray, and the proportion be- 
tween the amplitudes of vibration in the incident, the reflect- 
ed, and the two refracted rays. 
