DISPERSION OF LIGHT 45 



The beautiful play of prismatic colours, shown by many precious stones, and especially 

 by diamond, is quite independent of the colour of the stone itself, but is due to the 

 decomposition of white light into its coloured constituents by refraction within the stone. The 

 greater the dispersive power of a stone the more marked will be this play of prismatic 

 colours ; on account of the specially high power of dispersion of diamond, the play of 

 colours exhibited by this gem is far in advance of any other precious stone. 



This play of prismatic colours is sometimes, especially by English jewellers, referred to 

 as the " fire ■" of a stone. The same term, " fire," is, however, also used to denote the 

 brilliancy of lustre of a stone ; it was used in this sense above when dealing with the quality 

 of lustre. 



Any two facets of a cut stone which are not parallel may constitute a prism and thus 

 give rise to the decomposition of white light into its coloured constituents. The facets at 

 the back and front of a cut stone should be so related as to give the maximum 

 decomposition of white light. Further, the faces at the back of the stone must be steeply 

 inclined, so that light, entering the stone from the front and being resolved into its 

 component colours, will strike the back faces of the stone at such angles that it is totally 

 reflected by them and passes out again at the front of the stone. 



The more perfectly the form of cutting fulfils these conditions, namely, the greatest 

 possible decomposition of white light into its coloured components, and the greatest possible 

 internal reflection of this light from the back facets, the more beautiful will be the cut 

 stone. The form of cutting most suitable for ))ringing out the beauty of the diamond is 

 that known as the brilliant. This form is shown from different points of view in Figs. 29 

 and 52 among others, and in section in Fig. 20. 



The form of a brilliant will be discussed in detail later ; here it need only be mentioned 

 that its numerous facets give it approximately the shape of a double four-sided pyramid, of 

 which one apex is trunacted by a large plane, the table, and the 

 other by a smaller plane. A brilliant is placed in its setting so that 

 the table hn (Fig. 20) is at the front towards the observer, while the 

 small truncating plane hi is turned to the back away from the 

 observer. 



The path of a ray of light inside a stone cut as a brilliant is 

 shown in Fig. 20. Let us suppose a ray of light ab to fall on the 

 oblique facet M, and to be refracted within the stone in the direction 

 be. The refracted ray be falls very obliquely on the facet Jci, and ^ 



forms with the normal to this facet an angle greater than the critical Figs. 20. Path of a 

 angle of the substance ; it will therefore be totally reflected in the ""^^f "s^* ^° ^ ^"^" 

 direction ed, and cd and cb will be equally inclined to ki. In the 



same way the ray travelling along dc is again totally reflected from the surface hi in 

 the direction de, and is then reflected from the surface hn in the direction ef. The ray 

 travelling along ef strikes the facet Im at a high angle, that is, at an angle less than the 

 critical angle of the substance ; it is therefore possible for it to pass out of the stone into air 

 along the path^. This direction,^, will not, as a rule, coincide with the original direction 

 of the ray ab, since in its journey through the stone it has undergone two refractions and 

 three internal reflections. Moreover, as a consequence of the two refractions undergone by 

 the original ray of white light, ab, it will be split up into its component colours, and, on 

 emerging from the stone, will present to the observer a beautiful play of prismatic colours. 

 To avoid obscuring the diagram, the different paths of differently coloured rays are not 

 shown in Fig. 20, as they are in Figs. 17 and 18 ; the path, as shown in Fig. 20, may be 



