44 



GENERAL CHARACTERS OF PRECIOUS STONES 



colours of the rainbow, namely, red, orange, yellow, green, blue, indigo, violet. The 

 spectnam may be conveniently shown to a number of persons at once by placing a white 

 screen in the path of the coloured rays. 



Fig. 19 gives a perspective view of the path of rays of light from a candle-flame, A, 

 through the prism MNPM'N'P'. The ray of light AB falls on the face MNM'N' of the 



prism, and is resolved into the prismatic colours ; the red 

 ray travelling along BR, the violet ray along BV, and rays 

 of other colours between. These rays inside the prism meet 

 the face NPN'F, and on passing into the air are further 

 refracted and separated, the red ray taking the path RR', 

 the violet VV\ and so on. Since rays of light emanate 

 from every luminous part of the candle flame, a complete 

 image of it will be seen at A' by an eye placed at R'V. In 

 the direction FT the image will be coloured violet, that is, 

 the side v of the image nearer the refracting edge NN' of 

 the prism will be violet, while the margin r of the image 

 lying on the line R'R will be coloured red. To an eye 

 placed at R'V the image will be seen to the left of the 

 actual position of the object, as is shown in Fig. 19 ; the 

 image is thus nearer the refracting edge of the prism than 

 is the object. 



The length of the spectrum formed by a prism depends 

 upon a variety of conditions. It is longer the greater the 

 angle between the rays RR^ and W ; this, in its turn, 

 depends upon the dispersive power of the substance of the prism, for the spectra given under 

 similar conditions by two similar prisms, but constructed of different substances, will differ 

 in length. The amount of dispersion produced by a prism will obviously vary with the 

 difference between the degree of refraction of the red rays and of the violet rays ; the 

 difference between the refractive indices of a substance for red light and for violet light 

 is indeed frequently regarded as a measure of the dispersive power of the substance. 



Amongst precious stones, and indeed the majority of known substances, diamond has 

 by far the greatest dispersive power. The differences in the refractive indices of diamond 

 and of window-glass for red and for violet light, that is the dispersive power of these 

 substances, are given below : 



Red light, n = 2-407 

 Violet.light, n = 2-465 

 Dispersive power of diamond = 2-465 - 2-407 = 0-058. 



Fig. 19. Path of the rays of light 

 through a prism. (Perspective view.) 



For window-glass : 



Eed light, n = 1-524 

 Yiolet light, n = 1-545 

 Dispersive power of glass = 1-545 - 1-524 = 0-021. 



The dispersion produced by diamond is therefore more than double that produced by 

 window-glass ; as a result of this, the spectrum given by a prism of diamond will be more 

 than twice the length of that given by a prism of glass having the same refracting angle. 

 The prismatic colours are transmitted to the eye by diamond widely separated from each 

 other, and the stone owes much of its beauty to this fact ; in glass and other substances of 

 less dispersive power, more or Jess overlapping of the prismatic colours takes place, and this 

 renders them less perceptible to the eye as separate sensations. 



