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SIR DAVID BREWSTER ON CIRCULAR CRYSTALS. 611 
the other by a faint line of light, which, in some crystals, reaches the yellow of the 
first order. Beyond this is another annulus of pale blue light, divided, like the 
first, by a faint line. In some discs this annulus is divided into three, by two 
faint bands. Each sector of this annulus is subdivided by dark radial lines, into 
four or five spaces, and, sometimes, beyond this there is another annulus simi- 
larly divided, the tint of both being a white of the first order. 
The interesting fact in this description, and which will afterwards occupy our 
attention, is, that the two sharp black circular lines, or spaces, are wholly devoid of 
matter, and that the interior part of the disc is separated by them from the exterior 
part. 
Among the almost infinite variety of crystallisations which this substance pre- 
sents to us, I shall describe only another which, though we shall afterwards find 
it fully developed in other substances, occurs only in circular sectors of 30° or 45°. | 
It is represented in Fig. 11, in its complete state, and consists of a series of con- 
centric circles, composed of crystalline patches, which generally polarise tints not 
higher than the yellow of the first order. Each concentric circle appears at first 
to be separated from its neighbour, and each crystalline patch from those adjacent 
to it; but though this is in some crystallisations the case, yet in general, we can 
observe between the patches, in all directions, crystalline matter so exceedingly 
attenuated, that its existence is not made visible by its action on polarised light. 
2. Salicine.—In this substance, whether dissolved in water or in alcohol, I 
have found the most splendid circular crystallisations. They are generally very 
large, and their character is negative, like the rings in calcareous spar. When 
the crystals are small, and require a considerable power to be seen, their tint is 
the palest blue of the first order, but when their diameter is between the one-fifth 
and the one-thirtieth of an inch, and their tints those of the jist and second orders, 
they form, in the estimation of all who have seen them, one of the finest objects 
for the polarising microscope. 
One of the smaller crystals is shewn in Fig. 12, where the tint of the four 
sectors is blwish-white, while that of the circular rim is absolutely black, arising 
from the great thinness of the crystals which compose it. That they are trans- 
parent crystals and not opaque matter is proved in this, and in all similar cases, 
by turning round the analyser when the light freely permeates the rim, and has 
a slightly yellow tinge, being complementary to what Newron calls, in his Table 
of Periodical Colours, the Beginning of Black. 
A larger dise of Salicine is shewn in Fig. 13, where there is a sharp black cross 
in the centre, surrounded with five or six narrow and concentric black rings, which 
become white by turning the analyser; or we shall in future express it, in the 
white field. Beyond these central sectors, the black cross is wide and divergent. 
VOL. XX. PART IV. 8c 
