REFRACTION OF LIGHT 37 



and others. It is, however, liable to be modified in substances possessing certain properties ; 

 thus, should a substance possess a high refractive index and also a high dispersion, 

 the vitreous lustre will pass into adamantine lustre, which, again, may show an 

 approach to metallic lustre. Characteristic adamantine lustre is possessed only by 

 diamond, but an approach to it is shown by zircon, especially when colourless. 



Silky or satiny lustre is exhibited by minerals possessing a finely fibi'ous structure, 

 as, for example, the ornamental stone satin-spar, the fine green malachite, and the golden 

 tiger-eye. 



Pearly lustre is exhibited exclusively in those faces of crystals parallel to which 

 there is a direction of perfect cleavage, and then only when cleavage cracks have been 

 developed in the interior of the crystal. Topaz, felspar (the moonstone variety), and other 

 stones, sometimes show pearly lustre, but only on faces parallel to the perfect cleavage ; all 

 other faces have the usual vitreous lustre. 



Greasy lustre appears to be associated with the presence of numerous microscopically 

 small enclosures, which in many minerals are of constant occurrence. Elseolite, which is 

 sometimes cut for ornamental purposes, shows a typical greasy lustre ; other minerals, such 

 as olivine, of which the lustre is usually vitreous, may show an approach to greasy lustre. 



The lustre of some minerals, as, for example, turquoise, resembles that of wax, and is 

 described as waxy lustre. Others, again, have the lustre of resin : resinous 

 lustre is shown by many garnets, such, for instance, as hessonite, which, when massive, 

 sometimes closely resembles resin in appearance. 



3. Refraction of Light. 



The refraction of light, and the phenomena connected with it, have an important 

 bearing on the study of precious stones. 



We have already noticed that of the light which falls upon a transparent body, such 

 as a precious stone, a portion is reflected at the surface, while another portion enters its 

 substance and is propagated in straight lines through it. When the incident ray of light 

 strikes the bounding surface of the transparent body per- 



pendicularly, the light which passes into the interior of the , r .„ 



body is propagated in the same direction as that of the ^/ G\ ■-'/(_ 



incident ray. If, however, the incident ray strikes the surface / (;L.._.y/...^>-;^ 

 obliquely, the path taken by the light through the substance / ' \/ >^'''^ \ 

 of the body will not coincide in direction with the incident \ y^P ^ 



ray, but will be in a new direction ; the ray may then be said ■■. y^ / \ 

 to be bent or refracted. B, "./"'f 'f' .n 



In P'ig. 9 let MN be the surface of separation between ^'■-- •-.-l.-..---''' 



the transparent body (precious stone), S, and the air, L. A 



single ray of light travelling in air in the direction AC, and ^^<^- .^- Refraction of light on 



. \ . F .11 1 passing into a precious stone. 



striking the separating surface at C, will not be propagated 



in the stone in the same straight line, namely, along CK, but will be bent or refracted 



into the direction CB. CB is then the i-efracted ray corresponding to the incident ray AC. 



The directions AC and CB, and also DE, the normal to the surface at C, all lie in the 



same plane, which is perpendicular to MN, and in Fig. 9 is the plane of the paper. This 



plane is known as the plane of incidence. Whenever light passes from air into a stone, the 



refracted ray CB is always nearer the normal DE than is the incident ray AC ; that is, the 



light is bent towards the normal. This may be expressed otherwise by stating that the 



angle of incidence, A CD, is greater than the angle of refraction BCE. 



