1821.] Crystallized Bodies on Homogeneous Light, 1 ^ 



Although the constancy of the position of the virtual pole for 

 different thicknesses is sufficiently made out here, the small 

 differences which exist are certainly not attributable to errors of 

 observation, which, in the method 1 employed, are usually con- 

 fined within much narrower limits. They are due to minute 

 irregularities in the crystals themselves, consisting probably in a 

 state of imperfect equilibrium of the molecular forces of aggre- 



fation, to which this salt is so subject, that it is rather rare to 

 nd a specimen in which the rings beyond both poles have 

 exactly the same breadth or tints. 



IV. Of the Tints developed by Crystals with tioo Axes out of the 

 principal Section. 



If we place a crystallized plate at an azimuth zero in a tourma- 

 line apparatus, having the axes of the tourmalines at right 

 angles, we shall observe, if its thickness be at all considerable, 

 that the two oval spots on either side the axis of symmetry 

 (which is now perfectly black) instead of being exactly regular in 

 their figure, as in PL IV. fig. 2, and tinged with colours symme- 

 trically disposed on either side of a line m ii perpendicular to 

 the principal section, are invariably coloured at one extremity r 

 with a strong prismatic red hue, and drawn out at the other v 

 into more or less elongated and tapering spectra or tails of blue 

 and violet light. The extremities r, r of the rings too have a 

 large excess of the red rays, and the opposite t;, v of the violet 

 rays. In crystals of the first class above described, the red 

 extremity is turned towards the other pole, while in those of the 

 second it is directed from it. If we subject a plate of Rocliell^ 

 salt to this examination, the ovals a, ff, are drawn out to a sur- 

 prising length, and the whole prismatic spectrum is displayed in 

 them with great vividness of colour, while the violet portions of 

 the rings are greatly elongated also, and appear to run into one 

 another. If the plate be turned round in azimuth, the phenomena 

 assume the most singular appearances of distortion ; and as the 

 rotation approaches to 45°, the rings in the vicinity of the pole 

 are gradually obliterated by their mutual ^ overlapping, Vvhich is 

 the greater the thicker the plate. In all situations, however, the 

 interposition of a red or dark green glass immediately restores 

 the perfect symmetry and regularity of the rings, which are then 

 seen in much greater number, and completely well defined. 



All this is the necessary consequence of the want of coinci- 

 dence of the axes for different colours. The lateral spots^ for 

 example, are formed for each homogeneous colour with perfect 

 regularity close to their corresponding pole, and regularly 

 decreasing in size from the red to the violet. Their arrangement 

 will, therefore, be as represented in fig. 3, R, O, Y, &c. being 

 the poles corresponding to the several colours, red, orange. Sic. 

 The oval spots composed of red rays being represented by r, r, 

 those of the other colours will be super-imposed on thera in their 



l2 '^ 



