Brewster's Luminous Figures. 501 



in nitric acid or not ; and hydrochloric and nitric acid can be 

 distinguished by means of the figures. In such corrosion, 

 microscopic triangles are formed upon the rhombohedral surface, 

 one point of which is directed towards the vertical solid angle, 

 and therefore opposite the ray r. These triangles arise from 

 depressions which correspond to a three-sided pyramid (the 

 vertical top-piece of a rhombohedron). 



In transmitted light the phenomena are as follows : — When 

 one face was corroded with hydrochloric acid, a star was seen con- 

 sisting of three pencils of light becoming broader on the outside ; 

 when at the same time the opposite parallel face was corroded, 

 the star appeared six-rayed. If two parallel faces were corroded 

 with nitric acid, on looking through the crystal an acute-angled 

 cross was seen, with spots of light in the obtuse angles. The 

 deportment of the so-called Streifenspath is also interesting : it is 

 a hemitrope of a rhombohedron in oscillatorial repetition, where 

 the face of rotation is — JR. One surface of the cleavage-rhom- 

 bohedron is striated in the direction of the long diagonal, the 

 rest are smooth. If a candle-flame is looked at from some 

 distance through the latter faces, by holding the principal 

 section (through the vertical solid angle) in a vertical position, 

 rhomboids appear like fig. 9, whose point of intersection show 

 the flame partially with prismatic colours. Through the striated 

 surface this face is only seen distorted. If such a crystal is cor- 

 roded, the smooth faces also appear corroded like fig. 10, face 

 b and c ; and on looking through against a flame, a band of light 

 appears cutting these lines at right angles, in which the flame 

 appears in spots of light at equal distances. 



Volger assumes that all primary forms of calcite are triplets, 

 through a threefold repetition of the above law. The optical 

 deportment does not agree with this ; for the crystals in which 

 the hemitropic aggregation mentioned is distinctly visible, ex- 

 hibit through the basal faces in polarized light entirely peculiar 

 phenomena, which do not occur in the ordinary fundamental 

 forms of calcite. I have described these phenomena in the 

 Miinchner Gel. Anz. 1855, No. 18. 



When the hexagonal prism of calcite is corroded by several 

 immersions in dilute hydrochloric acid, fig. 11, PI. III. appears on 

 alternate faces, like 1 and 2 ; the ray going parallel to the axis is 

 turned both above and below towards the vertical eds;es of the 

 cleavage-rhombohedron. I observed these images in reflected 

 light in two crystals from Andreasberg an inch long. When a 

 face on the cleavage-rhombohedron of calcite is ground smooth 

 by rubbing it with circular motion on a broad file, and then the 

 face cleaned with water and dried, on looking through it at a 

 flame a luminous line is seen in the direction of the short dia- 



