Brewster's Luminous Figures. 503 



In formate of strontia there is a cross in the direction of the 

 diagonals of the rectangular tabular crystals. In the tabular 

 crystals of ferridcyanide of potassium, when corroded by being 

 breathed upon, there appears a beautiful oblique luminous cross 

 towards the junction edge with the pyramid, and a band at right 

 angles to the axis like fig. 15. By further careful corrosion 

 fig. 16 appears. On the surface small rhombohedra are visible 

 in the position indicated by tigs. 15 and 16. The luminous 

 figures are seen of especial beauty in incident light when the 

 crystalline lamina is held in a blackened piece of paste- 

 board. 



In the klinorhombic system, I could observe beautiful crystals 

 of sulphate of magnesia and ammonia. The lateral faces of the 

 prism of 109°, 12° show the reflected images Plate IV. fig. 17, 

 and in the following order : — the faces 1 and 2 adjacent to the 

 klinodiagonal principal section on the front side of the hendyoe- 

 hedron (hence the terminal face inclined towards the observer) ; 

 the parts of the cross a inclined upwards towards the obtuse lateral 

 edge on the terminal face, but b inclined downwards; in like man- 

 ner, but turned in front, these crosses show themselves upon the 

 surfaces 3 and 4 at the back of the hendyoehedron. The iso- 

 morphous compounds, sulphate of nickel and ammonia, protosul- 

 phate of iron and ammonia, sulphate of nickel and potash, and the 

 corresponding cobalt-salt, exhibit a similar deportment. 



In a very beautiful crystal of sulphate of manganese and am- 

 monia, the right half of the cross-arm c in the face 2 was shorter, 

 and limited by an elliptical spot ; the left arm of the cross cor- 

 responding to face 3 was similar. Sulphate of oxide of copper and 

 potash showed this figure of reflexion but indistinctly. 



When a plate of gypsum is dipped for a few days in water, or 

 for a shorter time in dilute hydrochloric acid, the perfect cleavage 

 plate in reflected and transmitted light shows a beautiful band of 

 light, rectangular or almost rectangular to the cleavage-face, 

 which is characterized by the conchoidal fracture fig. 18. 



In the klinorhomboidal system, I observed sulphate of copper, 

 fig. 19. With a very slight corrosion there was exhibited on the 

 face p' a cruciform ray of light, fig. 20; on p a band of light 

 (fig. 21) rectangular to the edge of the prism ; and on the terminal 

 face o the reflected image fig. 22 ; in a certain direction the 

 trefoil inclined towards the solid angle ee. These images were 

 observed in two very beautiful crystals with smooth faces ; in 

 general the faces of this crystal are not smooth enough. 



I have here only described the faces in which the luminous 

 figures are distinctly shown ; in many salts which I further 

 investigated, for instance green vitriol, sulphate of magnesia, 

 white vitriol, chromate of potash, saltpetre, &c, I could not 



