POLARISING STRUCTURE OF DOUBLY REFRACTING CRYSTALS. 283 



Similar experiments were made with Quartz, a crystal of the 

 positive class, and with various other crystals, both with one and 

 two axes of double refraction ; and in every case the tints were 

 either raised or depressed in the scale of colours. The same 

 effects were obtained at various angles with the axis, either by 

 reducing the thickness of the plates, or by bringing the tints 

 within the limits of Newton's scale, by the opposite action of 

 plates of sulphate of lime. 



We may therefore consider it as an established fact, that the 

 phenomena produced by the polarising forces of all crystals, 

 whether they have one or more axes, and whether their action 

 is positive or negative, are very considerably affected by sub- 

 jecting the crystals to compressing or dilating forces. 



The effect which we have now described may arise from 

 two causes, either from an actual modification of the original 

 polarising force of the crystal, or from the developement of a 

 new force, which merely combines its effects with those of the 

 original force. The first of these cases is exemplified, when 

 we subject to pressure a plate of glass along which heat is in 

 the act of being transmitted. The pressure which is thus ap- 

 plied, alters the state of aggregation into which the glass is 

 thrown by the passing heat, and produces a real modification 

 of its former polarising force. When, on the other hand, we 

 combine a plate of sulphate of lime with a plate of calcareous 

 spar, the resulting tint arises merely from a combination of 

 the tints which these crystals produce separately ; the po- 

 larising force of the calcareous spar remaining the same as be- 

 fore. 



In order to determine whether pressure modifies the origi- 

 nal force, or creates a new one in doubly-refracting crystals, 

 I cut the crystals into different shapes, and found that the ef- 

 fect produced by pressure varied with the external shape of 



N n 2 the 



