119 
tocMhited by Mother-of-Pearl. 
parallel to each other, and in lines nearly straight along the 
whole surface, and the alternate ones at least must be regular 
crystals, having their optic meridians parallel to their own 
planes, and their optic axis (by which I mean the axis of 
symmetry in their spheroid of double refraction) perpendicular 
to the natural surface. The intermediate laminae, if composed 
of the same substance, must have their axes inclined to those of 
the former at some determinate angle, depending probably on 
no very remote laws of crystallography. The grooved appear- 
ance above described, may possibly arise from an actual diffe- 
rence in the resistance of these two sets of laminae to the action 
of the polishing particles, and therefore consist in a real diffe- 
rence of level ; but this I much doubt, from the simple faci, 
that I have never been able to transfer their impression to other 
transparent bodies, such as melted rosin, shel-lac, balsam of 
Tolu, &c. though in ail cases the first set of grooves, however 
fine, has been transferred with the utmost fidelity, and the 
7700th part of an inch, though a very minute quantity to our 
senses, appears to me enormously too wide to oppose the free 
introduction of a fluid under such circumstances. It is more 
probable, that the appearance is a mere optical illusion, though 
a most complete one, arising from the difference of action of the 
contiguous surfaces on the light transmitted from below. 
The regularity of structure here supposed, is not at all in- 
compatible with the irregular and arbitrary disposition of the 
grooves described by Dr Brewster. These are the intersections 
of the plain artificial surface with the thin coats deposited in 
succession by the living animal, which, though laid symmetrically 
on each other, like the laminae of mica, have yet a slight de- 
gree of irregular curvature, and a small and varying inclination 
to the polished face. Their form and breadth is regulated by 
this curvature and inclination, like the level lines traced by a 
receding tide on a slightly inclined sea-beach, or those on the 
surface of a wooden board, where its concentric layers rise in 
succession at different angles to the surface. Indeed, the face of 
an ordinary deal plank, cut at some distance from the centre of the 
tree, however coarse the simile may appear, when smoothed by 
planing, and afterwards subjected to the friction of rough particles, 
as in a floor, is a lively and faithful representation of the surface 
