
SIR DAVID BREWSTER ON CIRCULAR CRYSTALS. 621 
It is equally difficult to assign any reason for the production of the concentric 
bands of a uniform tint, which suddenly pass to another tint belonging to a 
different order of colours, and produced by a different thickness of material. A 
circular ring of green, for example, will pass per saltum, to a red of the next order, 
from a thickness of 9 to a thickness of 18; and this, according to a law which 
operates at every point of the circumference of the ring. Nor is this phenomenon 
less remarkable when the transition takes place in the very lowest order of tints, 
and at the smallest thickness of the film, as shewn in Figs. 6, 10, and 13, where 
the tint passes in repeated alternations from the pale blue to the beginning of 
black, rising to a maximum of blue, and again descending to the minimum of 
black. 
The black rings or circles shewn in Figs. 5, 6, 10, and 13, require to be care- 
fully studied, and with the finest microscopes. In most cases they seem to be 
spaces devoid of crystalline matter; but they have in general another origin. A 
line often appears perfectly black, when it corresponds with the violet of the 
_ second order, which separates the indigo of the same order from the red of the 
Jirst order. Another set of lines appear black, from their being the junction lines 
of crystals not in perfect optical contact. A third set of black circles are pro- 
duced by the extreme thinness of the substance, which is not capable of polarising 
the very black of Newton’s scale, and the existence of which upon the glass plate 
can be ascertained only by the highest powers of a fine microscope. But though in 
all these examples there is no breach of continuity in the circular disc, yet there 
are cases, as in the double black ring in Fig. 10, where the corresponding space is 
devoid of all crystalline matter. The crystallisation of the disc had been completed 
at the inner margin of the first black ring, and by some repulsive power the 
molecules in the solution were kept at a distance from the completed disc, and de- 
posited themselves in a scarcely visible ring around the outer margin of the first 
black ring. The repulsive power again came into play; and another black ring 
intervened, the molecules being deposited at the same distance as formerly from 
the last-formed ring. What repulsive power this is, if it is not electrical, and 
how it operates, if it is electrical, we cannot even conjecture. 
Another remarkable peculiarity in circular crystals is shewn in Figs. 11 and 
12, where, as in chromic acid, the disc consists of alternations of dark and lumi- 
nous circles, equidistant from each other. The dark circles are composed of the 
acid in particles too small to polarise light, and the luminous ones of separate 
patches of crystalline matter thick enough to give the b/we and sometimes the 
white of the first order, and separated from one another by matter too thin to 
polarise light. In some rare cases, the spaces between the circles and between 
the patches are, like the black rings formerly described, devoid of crystalline 
matter. The separation of the patches in this case, is no less remarkable than 
the separation of the luminous circles. In the Adipocire from Paris, the tint of 
