POLARISING STRUCTURE OF DOUBLY REFRACTING CRYSTALS. 285 



Table, shewing the Effect of Compression and Dilatation 

 upon Positive and Negative Crystals* 



Positive crystals, compressed, 

 dilated, 



Negative crystals, compressed, 

 dilated, 



Axis of Compression or 

 Dilatation parallel to 

 the Axis of the Crys- 

 tal. 



Tints rise, 

 Tints descend, 



Tints descend, 

 Tints rise, 



Axis of Compression or 

 Dilatation perpendicu- 

 lar to the Axis of the 

 Crystal. 



Tints descend. 

 Tints rise. 



Tints rise. 

 Tints descend. 



Now, since every compression is accompanied with a dilatation 

 in a direction perpendicular to the axis of compression, and 

 vice versa, it is obvious from the Table, that these simultane- 

 ous changes in the state of aggregation of the particles, will act 

 in combination, that is, they will conspire in producing either 

 a positive or a negative polarisation. 



The preceding results, which are deducible, a priori, from 

 the principles already established, I have confirmed by direct 

 experiments upon calcareous spar, quartz, and various other 

 doubly-refracting crystals, cut into different shapes ; and I have 

 obtained analogous results, by the application of pressure to 

 crystals with two axes of extraordinary refraction. 



If the axis of compression is perpendicular to the axis of 

 double refraction, the crystal is converted into a crystal with 

 two axes, the poles of the two resultant axes or diameters of 

 no polarisation being distinctly visible. 



The 



