DOUBLE REFRACTION OF LIGHT 47 



rarity and costliness. It is also possible by this means to distinguish glass imitations, which 

 are always singly refracting, from genuine precious stones, which are for the most part doubly 

 refracting. 



The kind of refraction, single or double, exhibited by a body is a necessary consequence 

 of the ciystalline structure of its substance, and varies in the different crystal systems. All 

 amorphous bodies, together with all those which crystallise in the cubic system, are singly 

 refracting, while all other crystals, without exception, namely, those included in the 

 hexagonal, tetragonal, rhombic, monoclinic, and triclinic systems are doubly refracting. It, 

 is thus possible fi-om the behaviour of a stone with respect to the refraction of light ta 

 learn whether, on the one hand, it is amorphous or crystallises in the cubic system, or 

 whether, on the other hand, it crystallises in one of the five remaining crystal systems ; and 

 this observation can be made on a very small irregular fragment of the mineral. Thus in 

 the example just quoted we know that the singly refracting spinel must crystallise in the 

 cubic system, while the doubly refracting ruby crystallises in one of the remaining five 

 systems, namely, the hexagonal. 



Since the observation of the kind of refraction, whether single or double, exhibited by 

 a stone is a step towards determining to which of the crystal systems it belongs, and more- 

 over is frequently a decisive test of its identity, it is important to be acquainted with the 

 method of making this observation. In the third part of this book, dealing specially with 

 the determination of precious stones, considerable use will be made of this method, and it 

 will also be mentioned under the description of each species of precious stone. 



In some substances the phenomenon of double refraction is directly observable, for an 

 object, when viewed through a plate of the substance, will appear double instead of single^ 

 as is more usually the case, for example with a plate of glass. 

 Each of the two refracted rays BO and BE (Fig. 21) gives 

 an image of the object ; these two images are, as a rule, very 

 close together, but in some few minerals they may be so widely 

 separated as to be both distinctly visible. 



In Fig. 22, let MNPQ be a plate of doubly refracting 

 substance with the surface MN parallel to the surface PQ. 

 The incident ray of light AB, striking the surface MN at B, 

 enters the plate and is split up into the two rays BO and 

 BE; these emerge from the surface PQ in the directions 00' 

 and EE' both parallel to AB. Each of these rays 00' and ^^^^^ Path of light through 

 EE' gives rise to an image of the source of light, and an eye a doubly refracting plate. 

 placed at O'E' will see one image along 00 and another 



along E'E. Other conditions being equal, these two images will be the more widely 

 separated the thicker the plate is. 



A substance which shows the phenomenon of double refraction to a very marked degree 

 is calcite or Iceland-spar, which on this account is also called doubly refracting spar. If a 

 crystal, or, better still, a transparent cleavage rhombohedron of Iceland-spar is placed over 

 an object, such, for instance, as the page of a book, the letters, vhen viewed through the spar, 

 will appear double, as shown in Fig. 23. 



In calcite the two refracted rays are inclined to each other at a comparatively large angle, 

 much greater than in the majority of other minerals. The greater the angle of separation 

 of the two refracted rays {OBE in Fig. 21) the greater the double refraction of the mineral, 

 and different substances differ considerably from each other in this respect. 



The double refraction of the majority of precious stones is not very strong; and as 



