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, 1 made a considerable number of such plates with different kinds 

 of wax, and with various proportions of resin, and 1 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- 

 tions. 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 ex- 

 hibits 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 tvi^o plates of glass, it shows the qua- 

 quaversus 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- 

 rounded 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 re- 

 ceived from pressure, I took a few drops of the melted compound 

 and placed them in succession 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 glass, I squeezed out 

 ■the drop into a thin plate. This plate was now almost perfectly trans- 

 parent, as if the pressure had brought the particles of the substance 

 into optical contact. 



Vol. XXI.— No. 2. 38 



