610 SIR DAVID BREWSTER ON CIRCULAR CRYSTALS. 
orders of tints, by compensating them with the corresponding tints of plates 
of selenite. 
In other discs the rings 2 and 3 have each the same colour throughout,—the 
one generally. ved and the other green, and having no relation, either to the central 
tint at 1, or to one another. In some cases the order of colours is completely 
inverted, as in Fig. 4, where the central tints are a b/ue of the second order, gra- 
dually passing through red and yellow to a brilliant white of the first order. In 
other crystals I have found the central tints red, green, and yellow, of high orders; 
but in these cases the discs are not regularly formed, and the elementary crystals 
not wholly in optical contact. 
The most perfect circular crystals are those in which the central tints are the 
blue and white of the jirst order. This arises from the extreme minuteness of the 
crystals, which thus form a more uniform disc, and cause the black cross to have 
a degree of sharpness, which it requires a considerable magnifying power to ex- 
hibit. In such crystals, the central portion is surrounded with a black and narrow 
ring, beyond which there is another annulus of sectors, sometimes white like the 
inner ones. This is again terminated by a black circle, beyond which is a third 
series of sectors, sometimes white and sometimes a d/ue of the first order. This 
structure is shewn in Fig. 5, where the black cross starts into different breadths, 
in passing from one set of sectors to the others,—an effect which is produced by an 
inferior degree of optical contact in the elementary crystals of the outer sectors. 
An interesting structure is shewn in Fig. 6, where all the tints are white, the 
central ones terminating in a dark circle, beyond which are four large sectors, 
whose tint is the bluish white of the first order, lower than the central tint. Each 
of these sectors, however, is divided into four portions by very faint circular lines, 
which scarcely depolarise the incident light, the tint being there a minimum, and 
increasing to the middle point between them. 
In Figs. 7 and 8, we have represented structures consisting of crystals, shoot- 
ing out, as it were, from the centre, and all of a golden-yellow colour. In Fig. 7, 
the black cross is seen, but in Fig. 8 there is such imperfection of contact be- 
tween all the radial crystals, that the darkness of those under the black cross 
is scarcely visible. 
In discs like Fig. 7, a very singular effect is sometimes produced, as shewn in 
Fig. 9, where the black cross is so divergent and wide, that the golden-coloured 
crystals half-way between its branches, have the appearance of a yellow rectan- 
gular cross. 
Under favourable circumstances, the discs assume a very interesting and com- 
plex appearance, as shewn in Fig. 10. Beyond the central golden-yellow radia- 
tions is a broad annulus of pale blue of the first order, divided by a faint, dark, and 
narrow band, scarcely luminous. This annulus is surrounded by a sharp and 
broad line, perfectly black, which is succeeded by a similar line separated from 


