crystallized bodies on homogeneous light. 73 
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 I employed, are 
usually confined within much narrower limits. They are 
due to minute irregularities in the crystals themselves, con- 
sisting, probably, in a state of imperfect equilibrium of the 
molecular forces of aggregation, to which this salt is so sub- 
ject, that it is rather rare to find a specimen in which the rings 
beyond both poles have exactly the same breadth or tints. 
Art. IV. Of the tints developed by crystals with two axes out of 
the prmcipal section. 
If we place a crystallized plate at an azimuth zero in a 
tourmaline apparatus, having the axes of the tourmalines at 
right angles, we shall observe, if its thickness be at all con- 
siderable, 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 Fig. 2. PI. V, and tinged 
with colours symmetrically disposed on either side of a line 
mn 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 ex- 
tremities r, r of the rings too have a large excess of the red 
rays, and the opposite v, 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 Rochelle salt to 
MDCCCXX. L 
