612 SIR DAVID BREWSTER ON CIRCULAR CRYSTALS. 
The whole of this annulus, which forms the greater part of the disc, is composed 
of crystals radiating from the centre, and of inequal thickness in their breadth, so 
that we have the luminous sectors not of one colour, as in the Lithoxanthate of 
Ammonia, but of various tints from white of the first to blue of the second order. 
The radiating crystals are sometimes sectors of 10° or 15°, of uniform thickness, 
and giving the same colour; and hence, the black cross is composed of sectors of 
different degrees of blackness as they are brought into the plane of primitive 
polarisation. Beyond this annulus, the disc terminates in a rim, like that of a 
carriage-wheel, composed of two or more concentric circles, between which the 
crystals are disposed in radial lines, sometimes not in optical contact, but exhibit- 
ing the same colours as those in the larger annulus. In discs of a considerable 
size, there are seen exceedingly minute and dark circles, about ten or twelve in 
‘number, which I have found to be cracks or lines of cleavage, and which are ac- 
companied with short lines of cleavage, passing radially from the one to the other. 
In these discs, there is another peculiarity which deserves to be noticed. In 
the coloured sectors, there are often circular spots and rings, in which the tint 
descends to z¢70, as if a drop of some solvent had fallen upon the crystal: and 
there are spots of an opposite kind, where the tint rises from that of the sector 
to higher tints, an effect probably produced by a particle of the crystal forming 
around itself, while dissolving, a thicker film, becoming thinner as it recedes from 
the particle. 
In some of the circular discs of Salicine, I have found the outer rim as wide as 
the interior portion, and in this case it polarises a blwish-mwhite of the first order ; 
but, what is peculiarly worthy of notice, this rim is subdivided by faint concen- 
tric rings of different degrees of darkness, into, sometimes, twelve or fifteen annuli 
of different degrees of brightness. This seldom takes place in the interior portion 
of the disc, but when it does occur, and the tints are brilliant, the subdivison of 
the annulus into a number of concentric circles of different colours is singularly 
beautiful. 
3. Asparagine.—The circularly polarising discs which this substance displays, 
resemble very much those of Salicine. They are more varied in their structure, 
and more beautiful in their tints. The rims of the discs are more highly coloured, 
and more uniform in their texture; and the concentric tints, whether they are all 
of different degrees of whiteness, or of higher orders of colours, are so perfectly 
regular, and so sharply defined, that the observer stands before them in mute ad- 
miration, and feels himself unable either to describe or to draw them. There are 
two peculiarities, however, which deserve to be noticed; the one, the existence of 
discs in which there is no circularly polarising structure; and the other, of discs 
exactly resembling, in the succession of black and white narrow rings, the systems 

