OPTICAL I'Uol'KHTIl.s OF CRYSTALS 



215 



1. !.')<) 1. 17."i . . Mixtures of petroleum and turpentine. 



I.IMI 1.535. . Turpentine and ethylene bromide. 



1 .")(() 1.635 . . Clove oil and a-monobromnaphthalem- 



(Mnonoohkmiaphthalene. 



i.titio 1.740 . . oe-monobrpmnaphthalene and methylene iodide. 

 1. 715-1.790 . . Methylene iodide and sulphur. 



When the index of refraction of the crystal fragments are the 

 >ame as the fluid in which they are mounted, the outlines of the 

 fragments are very indistinct and scarcely visible. This is well 

 illustrated by the glass rod in the Canada balsam bottle, the indices 

 of which are nearly the same, and the rod is scarcely visible. Quartz 

 mounted in balsam is also indistinct, as the indices <a = 1.544 and 

 c = 1.553, while for balsam n = 1.548, just between. In a section 

 of quartzite where tin- 

 grains are all the same 

 color, their outlines will 

 be invisible in ordinary 

 light ; but in crossed nic- 

 ols, where they will be 

 differently illuminated, 

 they are quite distinct, 

 Fig. 366. The outlines 

 of mineral fragments are 

 more marked or distinct, 

 the greater the difference 

 between the indices of 

 refraction of the two 

 sul istances. In calcite, 

 where = 1.486 and <o = 

 1.658, mounted in balsam the outlines are distinct and the surface 

 of the section is rough, Fig. 367. It is said to have a marked or 

 high relief. With the polarizer only in, if the section is revolved 

 through 90, the relief for the two rays will be noted to be quite 

 different, as the difference in one case is .11 and in the other 

 .062. The relief then is a measure of the difference of the index 

 of refraction of the mineral and the index of refraction of the 

 medium in which the section is mounted. 



VI. Becke's method, or the determination of the relative 

 values of the indices of refraction of two adjacent minerals in the 

 same section. Where the index of refraction of one is known, that 



Fiu. 367. Section of Calcite showing Marked 

 Relief and Rhombohedral Cleavage. 



