DOUBLE REFRACTION OE LIGHT 51 



will be found that in certain positions it does not allow the light reflected from s.s to pass 

 through, and in these positions the field of view becomes dark, while in other positions it is 

 light. On tui-ning the NicoFs prism through a complete revolution, that is 360°, it will be 

 observed that there are four gradual changes from maximum lightness to maximum darkness 

 or vice versd ; the four positions of maximum lightness and maximum darkness being 

 separated by angles of 90°. If the NicoPs prism be turned into one of the two positions in 

 which the field of view has maximum darkness, the polariscope will afford a means whereby 

 singly and doubly refracting stones can be distinguished from each other with ease and 

 certainty. 



The different behaviour of singly and doubly refracting substances when examined with 

 the polariscope is as follows : When a singly refracting substance, such as a piece of glass, 

 is placed on the object-carrier, no, and observed through the NicoPs prism, the whole of the 

 field of view will be dark and will remain dark during the rotation of the object- carrier. It 

 should be mentioned here that in this, as well as in all other observations, it is advisable to 

 shade the side light from the object with the hand, or, better still, by means of a tube of 

 black paper placed on the object-carrier and round the object ; otherwise light will reach 

 the eye which has been reflected from the surface of the stone without passing through it. 



When a doubly refracting body is examined in the same way, it is found that in certain 

 positions the portion of the field which it occupies becomes light. This is due to the fact 

 that the polarised light, which before was unable to pass through the Nicol's prism, becomes 

 so modified by its passage through the doubly refracting substance that it is now capable of 

 passing through the Nicol's prism, when it will reach the eye of the observer. As the object 

 is turned round through 360°, there will be eight changes from maximum lightness to 

 maximum darkness or vice versd ; there being four positions of the stone in which the 

 lightness is a maximum, and four in which there is maximum darkness, an interval of 45° 

 lying between each. Through the complete rotation of the object, however, the portion of 

 the field not occupied by it remains dark so long as the Nicol's prism is undisturbed. 



There is thus an essential and important difference in the behaviour of singly and doubly 

 refracting stones when examined in polarised light. A singly refracting stone remains dark 

 in the dark field of the polariscope, while a doubly refracting stone changes from light to 

 dark as it is rotated with the object-carrier. 



Even in this method, however, there are certain liabilities to error which must 

 be carefully avoided. In all doubly refracting substances the strength of the double 

 refraction is not the same in all directions. Thus the two images of a flame or needle seen 

 through a doubly refracting stone wiU be further apart when viewed in some directions than 

 in others, while in certain directions a single image only is to be seen. The substance is 

 therefore, along these particular directions, not doubly refracting but singly refracting. 



Those directions in a doubly refracting body along which there is only single refraction 

 are known as optic axes. All doubly refracting stones can be grouped into two classes : 

 the one containing stones having one optic axis, described as being optically uniaxial ; 

 and the other containing stones having two optic axes, and described as being optically- 

 biaxial. The optic axes of any given substance are closely connected both in number and 

 direction with its crystalline form. Thus all hexagonal and tetragonal crystals are uniaxial, 

 and the optic axis of these crystals coincides in direction with the principal crystallographic 

 axis. All rhombic, monoclinic, and triclinic crystals are biaxial, and in the case of rhombic 

 and monoclinic crystals definite relations exist between certain crystallographic and optical 

 directions. Crystals belonging to the remaining system, namely the cubic, are, as mentioned 

 above, optically isotropic, that is, singly refracting. 



