On Double Refraction. 297 
The subject of double refraction was then so little developed that 
this experiment 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 colored rings at oblique incidences are produced 
by the inclination of the refracted ray to the axis of double refraction. 
In order to examine this remarkable effect under a more general as- 
pect, 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 direc- 
lions. If the film of wax is made extremely thin, the particles are 
ot sufficiently numerous to exhibit any action upon polarized light. 
When resin alone is melted and cooled in a similar manner, it ex- 
hibits no doubly refracting structure, whether it indurates slowly or 
under the influence of pressure. 
Ifresin and white wax are mixed in nearly equal proportions, the 
Compound possesses considerable tenacity. When a proportion of it 
Smelted and cooled between two plates of glass, it shows the qua- 
(eaversus polarization of bees’ wax, the axes of the elementary par- 
ticles being turned in every direction. It possesses a considerable 
degree of opalescence, and a luminous body seen through it is sur- 
tounded with nebulous light. This imperfect transparency evidently 
‘rises from the reflexion and refraction.of the rays in passing from 
*€ molecule to another, occasioned by a difference in the refractive 
Power of the ingredients, or by the imperfect contact of the particles, 
by both these causes combined. 
Ih order to observe the modifications which these phenomena re- 
‘tived from pressure, I took a few drops of the melted compound 
ind placed them in succession on a plate of thick glass, so as to form 
‘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 
"ertical pressure on the centre of the piece of glass, I squeezed out 
drop into a thin plate. This plate was now almost perfectly trans- 
Parent, as if the pressure had brought the particles of the substance 
0 optical contact. * 
Vou. XXI—No. 2. 38 
