REFRACTION OF LIGHT 39 



separation between the stone and the liquid. The amount of bending depends in each case 

 upon the difference in the refractive indices of the two media through which the ray passes. 

 Thus, if the refractive index of the liquid is much greater than that of air, the ray of light 

 will be much bent in passing from air into the liquid. Similarly, if the refractive index of 

 the stone is not much greater than that of the liquid, the ray will experience but little 

 bending when entering the stone ; while if the index of refraction of the stone and that of 

 the liquid are identical, there will be no bending of the ray of light, and it will travel 

 through both in the same straight line. 



The use of methylene iodide in determining specific gravities has already been 

 described. Another of its convenient properties is a very high index of refraction, the 

 value of which, Inoreover, can be diminished by diluting the liquid with benzene. If a 

 stone be immerseij in methylene iodide so diluted with benzene that the refractive index of 

 the liquid is the sa^e as that of the stone, there will be no bending of the rays of light, 

 and they will pass in straight lines through the liquid and the stone. Provided that the 

 liquid and the stone are of the same colour, the result will be that the latter becomes 

 invisible and cannot be detected. If the index of refraction of the liquid be changed by 

 the addition of benzene or of methylene iodide, the boundaries of the stone will become 

 visible ; its outlines will grow sharper and more distinct as the difference between its 

 refractive index and that of the liquid is increased by the further addition of either one or 

 other of the liquids. 



The phenomenon just described is sometimes made use of for the purpose of discovering 

 hidden cracks, enclosures, and other flaws in precious stones. The stone is immersed in a 

 strongly refracting liquid such as methylene iodide ; its external boundaries will then 

 become less distinct or, if the stone has the same refractive index as that of methylene iodide, 

 invisible. Any flaws in the interior of the stone will thus be rendered prominent and can 

 be easily seen . 



Light is refracted not only when passing from an optically rarer into an optically 

 denser medium, as, for instance, from air into precious stone, but also in the reverse case, 

 as, for example, when a ray of light in a stone passes out into the air. In the 

 passage of light from a denser to a rarer medium, the law of 

 refraction still holds good. We shall see from Fig. 10, however, 

 that the refracted ray is in this case bent away from the normal, 

 or, in other words, the angle of incidence is less than the angle 

 of refraction ; while in the previous case the refracted ray was 

 bent toioards the normal, and consequently the angle of incidence 

 was greater than the angle of refraction. 



In Fig. 10, let MN be the surface of separation between the 

 stone A' and the air L. It will be seen that the angle of incidence I" 



^CZ)of the ray ^C in the stone is less than the angle of refraction Fig. 10. Refraction of light 

 BCE of the refracted ray; also that the refracted ray BC is bent °° P^'''^"^ out of a precious 



1 • 1 1- <■ 1 stone. 



away from the normal. In this case also, the bending of the ray 



is greater the greater the index of refraction of the stone, but the amount of bending is 

 the same whether the light passes from stone to air or vice versa. In one case the light 

 travels in the direction ACB, and in the other in the direction BCA. 



In the case also of the passage of light from a denser to a rarer medium, the angle of 

 refraction increases with the angle of incidence. In Fig. 11, where MN is the surface of 

 separation between the precious stone S and the air L, the ray AC, incident upon the 

 surface MN a.i C is bent into the direction CB, J^C into the direction CB^, and so on. As 



