88 
DR. BREWSTER ON THE DOUBLE REFRACTION PRODUCED 
The subject of double refraction was then so little developed that this expe- 
riment excited no notice ; and it was only brought to my own recollection by 
the accidental appearance of the specimen itself. This depolarizing film has 
suffered no change by remaining fifteen years between the plates of glass. The 
vertical line along which it is destitute of the property of depolarization is a 
single axis of double - refraction ; and the coloured rings at oblique incidences 
are produced by the inclination of the refracted ray to the axis of double re- 
fraction. In order to examine this remarkable effect under a more general 
aspect, I made a considerable number of such plates with different kinds of 
wax, and with various proportions of resin, and I was led to results which 
seem to possess considerable interest. 
When the white wax is melted alone and cooled between two plates of glass, 
it consists of a number of minute particles, each possessing double refraction, 
but having their axes turned in all possible directions. If the film of wax is 
made extremely thin, the particles are not sufficiently numerous to exhibit any 
action upon polarized light. 
When resin alone is melted and cooled in a similar manner, it exhibits no 
doubly refracting structure, whether it indurates slowly or under the influence 
of pressure. 
If resin and white wax are mixed in nearly equal proportions, the compound 
possesses considerable tenacity. When a proportion of it is melted and cooled 
between two plates of glass, it shows the quaquaversus polarization of bees’- 
wax, the axes of the elementary particles being turned in every direction. It 
possesses a considerable degree of opalescence, and a luminous body seen 
through it is surrounded with nebulous light. This imperfect transparency 
evidently arises from the reflexion and refraction of the rays in passing from 
one molecule to another, occasioned by a difference in the refractive power of 
the ingredients, or by the imperfect contact of the particles, or by both these 
causes combined. 
In order to observe the modifications which these phenomena received from 
pressure, I took a few drops of the melted compound and placed them in suc- 
cession on a plate of thick glass, so as to form a large drop. Before it was 
cold, I laid above the drop a circular piece of glass about two thirds of an inch 
in diameter, and by a strong vertical pressure on the centre of the piece of 
