. 208. OPTICAL PROPERTIES OF MINERALS. 295 



l?ainbow Calcedony, shews similar colours but more faint, 

 and here they depend upon composition. 



Another remarkable property of certain minerals is, that 

 they shew different colours, if examined by transmitted light 

 in different determined directions, which demonstrates that 

 it is intimately connected with their forms and structure. 

 This property of minerals has been called their Dichroism. 

 Rhombohedral Tourmaline, prismatic Quartz, rhombohe- 

 dral and prismatic Talc-mica, are among the most distinct 

 examples. Several varieties of the first are nearly opaque 

 in the direction of the axis, while they shew different de- 

 grees of transparency, and different green, brown, and blue 

 colours, in the directions perpendicular to it. Prismatic 

 Quartz is blue in the direction of the axis, yellowish-grey 

 perpendicular to it. Prismatic Talc-mica is sometimes 

 green in the direction of the axis, and brown perpendicular 

 to this line, &c. The application of this property is greatly 

 extended by examining minerals in polarised b'ght, where 

 many minerals shew dichroism, which exhibit in common 

 light the same colour in every direction. 



The Tarnish consists in the alteration of the colour of a 

 mineral upon its surface. It is necessary to be acquainted 

 with this peculiarity of certain minerals, in order to avoid 

 confounding it with their real colours. Minerals with a 

 perfect metallic lustre are almost the only ones subject to 

 become tarnished ; in these it produces many shades of 

 bright colours, the further distinction of which, however, 

 is of very little use in Natural History. Several minerals 

 become tarnished in a very short time, if a new fracture 

 has been effected. Among these we observe native Ar- 

 senic. 



Simple minerals very seldom present more than one colour 

 at a time. There are, however, examples of the occurrence 

 of two colours, as in rhombohedral Corundum, prismatic To- 

 paz, rhombohedral Tourmaline, prismatic Disthene-spar, and 

 a few others. Compound minerals, on the contrary, are very 

 often variegated ; and the Delineation of Colours comprises the 

 figures which the different colours produce. It is super- 



