4i6 Dr. Brewster on ?iew Properties of light exhibited 
incident upon the mother of pearl from a fluid of the same 
refractive power, it is evidently unconnected with the form of 
surface. These masses of colour appear to have the same ori- 
gin as the colours of thin plates described by Newton. Even 
when the angle of incidence is the same, the crimson light 
appears at one thickness of the mother of pearl and the green 
at a less thickness, and the transmitted light consists of colours 
complementary to those of the reflected light. We are there- 
fore, in this case, presented with phenomena almost exactly 
the same as those of thin plates, though produced by plates 
of mother of pearl of considerable thickness ; — a subject which 
is well deserving of more attentive examination. 
IV. On a new Species of polarisation peculiar to Mother of Pearl, 
Having seen, in the course of the preceding experiments, 
so many deviations from the ordinary laws of optics, I sus- 
pected that mother of pearl might exhibit similar anomalies in 
the polarisation of light. This conjecture was immediately 
confirmed by the discovery of a remarkable property, which 
forms the connecting link between the phenomena of polari- 
sation, as effected by crystallised and uncrystallised bodies. 
In all doubly refracting crystals the opposite polarisation of 
the two images is invariably related to some axis or fixed line 
in the primitive form ; while in all uncrystallised bodies the 
polarisation is related to the planes of reflection and refraction, 
the reflected pencil being always polarised in an opposite 
manner to the refracting pencil. Thus, if AB, fig. 6, be a 
plate of glass, and Rr a ray incident upon it at the polarising 
angle, the reflected ray rS will be polarised in the same man- 
ner as one of the pencils formed by calcareous spar, and a 
small portion of the transmitted ray rT will also be polarised, 
