POLARISING STRUCTURE OF DOUBLY REFRACTING CRYSTALS. 285 
~Tasze, shewing the Effect of Compression and Dilatation 
upon Positive and Negative Crystals. 
Axis of Compression or | Axis of Compression or 
‘Dilatation parallel to Dilatation perpendicu- 
the Axis of the Crys- | lar to the Axis of the 
tal. © Crystal. 
Positive crystals, compressed, | Tints rise, Tints descend. 
dilated, Tints descend,| Tints rise. 
Negative crystals, compressed, | Tints descend, | Tints rise. 
~ —— dilated, 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 
