SIR DAVID BREWSTER ON CIRCULAR CRYSTALS. 613 
of rings seen round the star Capella, with annular apertures, and drawn by Sir 
Joun HErscHeEt.* 
4. Manna.—This substance gives fine circular crystals, which are negative, 
whether obtained from fusion or an aqueous solution. The crystals obtained by 
melting the Manna are the most perfect and beautiful. The intersection of the 
arms of the black cross is so sharp that it sometimes requires a considerable 
power to develope it, and the four minute sectors around it. Beyond this the 
crystals radiate uninterruptedly till they are stopped by meeting with other 
crystals, and the whole of them are joined together in a hexagonal mosaic pave- 
ment. The colours are very bright, varying from the white of the first to the 
blue and green of the second order, and there is a uniformity in the tints, and 
consequently in the shading of the black cross, which indicates great equality in 
the elementary prisms, and in the forces which keep them in optical contact. 
The discs are seldom found separate, and they have no rims, no annuli, and no 
concentric cracks. 
5. Disulphate of Mercury.—this salt, dissolved in nitric acid, gives no circular 
crystals by rapid cooling; but, when the solution is cooled slowly, it yields posi- 
tive circular crystallisations of a square form, as shewn in Fig. 14, which under- 
_go interesting variations. The rectangular cross is sometimes wanting, and is, 
as it were, replaced by black lines, which meet at the centre. These lines are 
sometimes black in the white field, and are then junction lines where the optical 
contact is imperfect. The greater number of the crystals in which these lines are 
more or less perfectly seen are rounded at the angles. Sometimes they are 
nearly circular, and the tint which they polarise is very little above the beginning 
of black of Newton’s Table. 
When the crystals are thicker, they exhibit a singular variety of forms, of 
which I have given a specimen in Figs. 15, 16, 17, and 18, the relation of which 
to Fig. 14, will be easily recognised. The crystals shewn in Figs. 16 and 18 
were obtained from a weak solution of the salt, and are very interesting. In the 
dark field of the microscope, we see only the brilliant golden-yellow border, and 
it requires a strong light and a very high power to discover, in the black interior 
of the square, minute specks of light equally diffused over its surface. By a 
slight turn of the analyser, we perceive the slightly darker diagonal cross shewn 
in Fig. 16. These squares are often wholly and uniformly filled up with crystals 
of the same tint as their outline; and occasionally only part of the square is thus 
occupied. The small and often shapeless crystals (occasionally oval and pear- 
shaped), which form the outline of the square in Fig. 16, and of the cross in Fig. 
* Treatise on Light, § 770, Figs. 155, 156, 157, Plate IX. 
