198 Prof. Kobe 11 on Aster ism and 



crystals and aggregates of crystals a perfectly closed parhelial circle 

 is seldom witnessed. But Professor Pliicker has a calcite which 

 even shows through the cleavage-planes two such circles or rings 

 of light, which touch in the image of the flame of the candle, and 

 which, according to the inclination of the crystals, are either seen 

 near one another or in one another. Babinet has ascribed such 

 phenomena to a fibrous structure, and to the corresponding 

 cleavageof the crystal. Yolger has noticed thatveryoften the junc- 

 tion-faces of a twin crystal are the cause, and that the asterism of 

 a striated external face often changes when such a face is ground 

 away and then the polished faces are looked through. Neither 

 of them mention the investigations which Brewster made on the 

 subject simultaneously with Babinet, and in which partly he 

 observed corroded faces of natural occurrence, and partly, by 

 superficial corrosion (Aetzen), or by rough polishing, made the 

 inner structure active for the light. In this manner Brewster 

 has examined crystals of topaz, garnet, amphibole, axinite, bora- 

 cite, Liparite, magnetite, amethyst, diamond, and by corrosion 

 crystals of calcite, alum, Liparite, apophyllite, acetate of copper 

 and lime, sulphate of potash, &c. 



In the corrosions, in w r hich he used water, hydrochloric acid, 

 nitric acid, and also hydrofluoric acid, he observed that, accord- 

 ing to the nature of the corroding agent, the figures are changed, 

 and that by mechanical grinding on a polishing-stone, or with a 

 rasp or file, similar figures are produced, though not pure, and 

 remarkably altered in position as compared with those formed by 

 corrosion. These figures appear in reflected light (of a candle- 

 flame), and also in transmitted light ; and if the corroded face is 

 printed in isinglass, may be investigated in transmitted light. 



Brewster has investigated more accurately only the crystals of 

 the tesseral, hexagonal, and quadratic systems ; he could obtain 

 no definite results for the rhombic, klinorhombic, and klino- 

 rhomboidal systems. 



The following observations may serve as a contribution to the 

 knowledge of this asterism. 



When crystal-faces are to be observed by corrosion, care is to 

 be taken that these faces are smooth and reflecting, and to begin 

 with the feeblest corrosion. For salts very soluble in water, I 

 have used the following process. I moistened a piece of fine 

 cloth with paper and left a part near it dry; I then laid the 

 crystal face upon the dry part, and moved it into the moist part 

 and immediately back again : according to circumstances, this 

 was repeated several times. The cloth is laid on a piece of glass. 

 The observation is made with a candle-flame, and best in a room 

 which is otherwise dark ; and the crystal is held between the 

 thumb and fore finger of both hands, close to the candle, in order 



