214 



MINERALOGY 



pers = T. A slide .with a fine scratch is placed on the stage and 

 the microscope very carefully focused on the scratch o. Then the 

 mineral section is placed over the scratch. The scratch will now 

 not be in focus, but will again come in focus by turning the fine 



adjustment of the microscope, so 

 as to lift the objective; o will 

 appear as if at o'; the distance 

 oo' which the scratch seems to 

 have been lifted will depend upon 

 the thickness of the section and 



w 



FlG 365 the index of refraction of the min- 



eral. The apparent thickness of 



the section do' is determined by measuring the distance oo' with 

 the micrometer of the microscope, and subtracting oo' from T. 



T 



T 



1 



Then,n = 



T-oo' 



or 



The actual thickness 

 The apparent thickness 



, for n = 



sm_i_ tan i 

 sin r tan r 



-, as for these small angles the ratio of the tangents is 

 T oo 



practically the same as that of the sines. This method is only an 

 approximate one and thick sections must be used. Ordinary rock 

 sections are too thin for accuracy. 



V. A convenient method of determining the index of refraction of 

 minerals in powder or very small crystals is to mount them on a 

 slide in fluids of different indices of refraction. A list of such fluids 



FIG. 366. Section of a Quartzite, showing the Low Relief. Crossed Nicols. 



which cover the range of the indices of refraction of most important 

 rock-forming minerals, as given by F. E. Wright, are : 



