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double refraction may be produced artificially by effecting certain 

 changes in the mechanical condition of hard and of soft bodies. In 

 all these case?, he observes, the phenomena are entirely different 

 from those of regular crystals ; and in none of them is the doubly- 

 refracting force a function of the angle which the incident ray forms 

 with one or more axes given in position. In the year 1815, he no- 

 ticed the depolarizing properties of a thin film of a mixture of resin 

 and white wax, compressed between two pieces of glass. Acciden- 

 tally meeting with the specimen which had originally been the sub- 

 ject of this observation, he found that after fifteen years it still re- 

 tained this property of depolarization, and was induced to pursue the 

 inquiry to which it led. He varied the proportions of the ingredients, 

 and observed in the different cases the modifications produced in the 

 phenomena by employing various degrees of pressure. He found 

 that, in every point, there existed an axis of double refraction, per- 

 pendicular to the plane of the film, and that the doubly-refracting 

 force varied with the inclination of the incident ray to this axis, just 

 as happens with all regular uniaxal crystals. He infers from his 

 observations, that the property of uniaxal double refraction is com- 

 municated to the molecules simply by the agency of pressure ; for in 

 all cases where pressure has not operated, the aggregate does not 

 exhibit this property. These effects are precisely the same as those 

 which would be produced by subjecting elastic spheres to a regular 

 compressing force, the axis of pressure becoming an axis of positive 

 double refraction ; while extension, on the contrary, produces a ne- 

 gative axis. 



From the consideration of the preceding facts, the author is led to 

 a very simple explanation of the origin and general phenomena of 

 double refraction in regular crystals. He considers this property as 

 not being inherent in the molecules themselves, but as resulting from 

 their compression, either by an extraneous force, or by their power 

 of inherent attraction of aggregation. The phenomena of crystal- 

 lization, and of cleavage, prove that the molecules of crystals have 

 several axes of attraction, or lines, along which they are most power- 

 fully attracted, and in the directions of which they cohere with dif- 

 ferent degrees of force. Guided by the indications of hemitrope 

 forms, and supposing the molecules to be spherical or spheroidal, it 

 is inferred that these axes are three in number, and at right angles 

 to each other, and that they are related in position to the geometri- 

 cal axis of the primitive form. In like manner, the phenomena of 

 double refraction are related to the same axis of the primitive form, 

 and may be all rigorously calculated by a reference to three rectan- 

 gular axes. The author pursues the consequences of these prin- 

 ciples in their application to various kinds of crystals. It follows 

 from this theory, that the forms of the ultimate molecules of crystals, 

 existing separately, determine, within certain limits, the primitive 

 form to which they belong ; while the doubly-refracting structure, 

 and the precise form of the crystal, are simultaneously produced by 

 the action of the forces of aggregation. These views receive a re- 



