500 Prof. Kobell un Asterism and 



former, by superficial corrosion, a rectangular cross is produced, 

 and in the latter a luminous band lying in the short diagonal of 

 the dodecahedral surface. These images are not changed by hy- 

 drochloric acid. The rectangular cross upon the hexahedral sur- 

 face shows itself parallel to the sides and to the diagonals of the 

 surfaces; the first remains rectangular even with obliquely inci- 

 dent light, the latter becomes acute-angled. Potash-alum, ammo- 

 nia-alum, and chrome-alum comport themselves quite in the same 

 way. The three-rayed star of the octahedral surfaces are also 

 often seen in natural crystals of Liparite and magnetite. 



If an octahedral face of Liparite is rubbed smooth upon a rough 

 broad file, and then the surface purified with water and dried, 

 the three-rayed star is also seen in incident light, the rays directed 

 to the angles of the triangle. On a hemitropic crystal of nitrate 

 ofstrontia an inch long, the star on the octahedron faces was like 

 fig. 4 (corroded with water), and the rays did not proceed at 

 right angles to the junction edge of the octahedral or cubic faces, 

 or to the angles of the octahedral face, but were at an acute 

 angle thereto. By repeated corroding with water the cubic faces 

 showed fig. 5. 



In the quadratic system, I distinctly observed, upon looking at 

 a candle-flame through the basal face of the tabular crystals of 

 the apophyllite of Passa, a luminous cross in the direction of the 

 diagonal ; likewise mferro cyanide of potassium when superficially 

 corroded by breathing ; in sulphate of nickel I saw fig. 6 on the 

 basal surface in reflected light. 



On the faces of the quadratic pyramid in phosphate of ammonia 

 and arseniate of potash, after slight corrosion with water, the 

 reflected image of a three-rayed star is seen ; the rays of which, 

 however^ do not proceed to the angles of the triangle, as in the 

 case of the octahedron, but towards the sides, and cut under 

 angles of two kinds, as do the normals to these sides. 



In the hexagonal system, calcite, by corrosion with hydro- 

 chloric and nitric acids, presents beautiful phenomena, which 

 Brewster has already partially described. The crystal is dipped 

 in the acid and then in the water, and dried with a soft piece of 

 linen. By immersion in hydrochloric acid (one volume of acid 

 to seven of water) the luminous figure 7 is obtained on the surface 

 of the cleavage-rhombohedron; the short ray r proceeding to 

 the corner edge often lengthens like the rest, by being corroded 

 in the manner described, and luminous rays result, becoming 

 broader outwards, which are seen with great beauty in trans- 

 mitted light. The change which fig. 7 undergoes when the 

 crystal is immersed in nitric acid (diluted with one volume of 

 water) is very surprising ; fig. 8 is then seen. By this figure it 

 may easily be recognized whether a crystal has been immersed 



