42 



GENERAL CHARACTERS OF PRECIOUS STONES 



Fig. 14. Path of a ray of light 

 through a plate with parallel sides. 



air, the ray is again refracted, this time away from the normal, and takes the path CF. 

 From the geometry of Fig. 14, it is easily seen that the second angle of refraction, FCE^, is 



equal to the first angle of incidence, ABD ; and that the paths 

 of the ray outside the plate, namely AB and CF, are parallel. 

 The direction of the ray on emerging from the plate is 

 therefore the same as the original direction, but its path has 

 been shifted a small distance, represented in Fig. 14 by B' F. 

 On observing a small object A through a transparent body 

 with parallel sides, it will be seen in very nearly the position 

 it really occupies ; this will not be the case however when 

 the bounding surfaces, MN, NP (Fig. 15), are not parallel. 



Let the bounding surfaces, MN, NP, of the transparent 

 body in Fig. 15 be inclined to each other at an angle MNP. 

 We are then dealing with the path of a ray of light through 

 a prism. As before, let the path of the ray of light incident 

 upon MN be AB ; on entering the solid it will be bent towards the normal GH, and will 

 take the path BD. On emerging into air, the ray will be bent away from the normal 

 KL and will take the path DE. The angle between the original 

 path of the ray in air and its final path is ACF, and this measures 

 the total amount of bendins it has undergone. The source of 

 light A, if observed through the prism, will appear not in the 

 position it actually occupies, but at some point on the line ECF 

 which makes with the direction AB an angle ACF. I'his angle 

 varies under different conditions. It will be greater the greater the 

 refracting angle MNP of the prism, and the greater the refractive 

 index of the substance of the prism ; it depends also upon the angle 

 of incidence, ^5G. But it has a certain minimum value which cannot 

 be diminished by either increase or decrease of the angle of incidence ; 

 this minimum value of the angle ACF is known as the angle of minimum deviation. 



In passing through a prism, the differently coloured constituents of white light are 

 separated, and we have the phenomenon known as dispersion. The beautiful appearance 



of many precious stones, and specially of diamond; is due to 

 their dispersion of light. The coloured constituents of white 

 light, from a source such as the sun or a lamp, differ not only 

 in colour but also in refrangibility or capacity for being 

 refracted. Thus, the refrangibility of red light is the smallest, 

 and that of violet light the greatest ; yellow, green, and blue 

 light occupy in this respect inteimediate positions in the order 

 in which they stand. 



It follows, then, that though we have hitherto spoken of 

 a substance as having a single refractive index, this is only 

 strictly true for monochromatic light, such as that given out 

 by a Bunsen flame, or a spirit-lamp flame coloured by the 

 vapour of either of the metals lithium, sodium, thallium, and 

 indium. If white light be used, the refractive index of the 

 substance will be different for each constituent of the light, that for red light being the 

 least and that for violet light the greatest. 



When white light passes through a prism, then, the red rays will be deviated or 



Fig. 15. Path of a ray 

 of light through a prism. 



Fig. 16. Dispersion of light. 



