and make our general observations on them at any time. We 
may press together or open tiie lamina; which have been j)artially 
separated, of gyi)sum for instance, in order to produce the colours, 
either concentrically or otherwise, and this may be repeated, or we 
may allow them to be stationary for future use or practice, and to 
serve as one of our resources for a natural proof or example, and we 
may even measure with some accuracy the space allowed in jiUites 
for the different tints. (See Dr. Young's excellent paper*.) Thus 
Tab. \. a,b, c, d, e,f, are representations of colours, that are or may 
be produced in the flaws of or betwixt the laminae of transparent 
stones, such as the diamond, gypsum, or selenite, mica, carbonate 
of lime, &c. (See Brit. Mn. tab. 2.) I have a specimen of mica 
showing a minute opening or division of 'the laminae of about the 
eighth of an inch in diameter, having a yellow centre, which I place 
first; as it succeeds, in the increasing order of density, next to light. 
It is surrounded by light blue, passing through purplish brown to 
very dark, when, by degrees, it again passes into faint brown-}- wilh 
a light margin, and then the stone appears to be close and solid sur- 
rounding it; PL 1. a. In another part of the same specimen, I 
have a red centre, to which succeeds the yellow, and the yellow is 
succeeded by the blue, and the other rings succeed in due order as 
the first ; PI. I. b. A third flaw has blue in the centre, as in c, with 
the red, yellow, and the usual rings in the same succession. It is 
not often that we can find one colour only shown in these sub- 
stances, they being more commonly a succession of rings. I have 
however met with dark brown with a lighter margin, passin"' to 
light or white as at d, and e shows the stone grayish in the centre 
with the white margin, surrounded by the brown and other rings, as 
it were reversed in order: it is in fact the curved margin of the va- 
rious and unequal divisions or fissures in the substance. In general 
the external or extreme mai-gin finishes with brown, as most of 
these, whether narrow or broad, do. The other more commonly 
* Phil. Trans. 1802. 
t Browns have scarcely, if at all, been considered by authors on this subject. 
c 
