Dr. Brewster on the Polarisation of Light. 229 
If two separate sets of polarising plates are placed before 
two luminous objects, in such a manner that their planes of 
refraction are perpendicular to each other, and that the rays 
are incident upon both parcels at the polarising angle belong- 
ing to each, the observer will perceive two images which have 
all the properties of those formed by doubly refracting crys- 
tals ; and in so far as the polarisation of the two pencils is 
concerned, the two parcels of plates form an artificial polari- 
sing crystal. For if these two images are viewed through a 
prism of Iceland spar, they will be converted into four images, 
and by turning round either the spar, or the plates, two of 
the images will vanish and reappear in every quarter of a 
revolution. 
From these observations, we are entitled to conclude, that 
the mystery of the polarisation of the two pencils formed by 
double refraction is completely removed by a reference to a 
more general principle, and that the formation of two images 
and the oblique refraction at a perpendicular incidence are the 
only parts of the problem which now require a solution. 
From the view which has just been given of the nature and 
consequences of the polarisation of light by oblique refraction, 
you will readily see that the prosecution of these experiments 
will lead to results of still higher interest. The establishment 
of a new law must at all times be considered as an important 
>step in the progress of science, but when this law presides 
over a class of facts, all of which are unexplained, and many 
of which still remain to be discovered, it claims a higher regard 
both as an instrument of discovery, and as a principle for ex- 
plaining new and analogous phenomena. 
If this paper shall meet with your apprebation, and that of 
