. 195. OPTICAL PROPERTIES OF MINERALS. 277 

 . 195. LUSTKE, COLOUR, TRANSPARENCY. 



The phenomena observable in minerals, with 

 respect to reflected and transmitted light, are com- 

 prehended under the heads of Lustre, Colour, and 

 Transparency. 



These subjects are treated of in Natural History, only 

 in so far as they allow some application to its own peculiar 

 purposes, of discriminating and describing minerals. Their 

 explanation belongs to Natural Philosophy. 



In order to employ lustre, colour, and transparency, 

 agreeably to the purposes of the Natural History of the 

 Mineral Kingdom, it is necessary to determine and to pro- 

 vide with certain denominations, those differences which 

 may be distinguished in these properties, in respect to both, 

 their kind and their degrees of intensity. This will require 

 us to fix a certain impression upon our mind, and always to 

 designate this impression with the same name, or to recal it 

 to our memory, whenever we read this name, or hear it ut- 

 tered. It is necessary therefore to have experienced these 

 impressions upon our own mind, and explanations or de- 

 scriptions cannot be successfully substituted in their place. 



An acquaintance with the colours occurring in the Mi- 

 neral Kingdom, with the different kinds of lustre, &c., may 

 be acquired from the consideration of bodies, which are not 

 minerals. It may indeed be effected in more than one 

 way ; but it seems to answer the purpose best, if we em- 

 ploy these bodies themselves, this being not only a more 

 sure, but also an easier way of fulfilling our intention. 

 To those who do not intend to go any farther than to become 

 capable of making use of the Characteristic for discriminat- 

 ing varieties occurring in nature, a very small number of 

 specimens will suffice, the choice of which is not attended 

 with any difficulties. 



The relative value we have to attach to the employment 

 of these properties, depends upon their generality and con- 

 stancy, and consequently upon the possibility of their 



