196 



ELEMENTARY CHEMICAL MICROSCOPY 



with the determination of the refractive index just as before. 

 The two values obtained will, in the case of uniaxial crystals, be 

 the indices e and co. When dealing with biaxial crystals in order 

 to use the values in Bolland's tables first set the crystal so that 

 its prism edge lies parallel to a plane passing through the short 

 diagonal of the polarizing nicol. Next determine the index for a 

 position at 90 degrees to the first. If a third value can be found, 

 determine it If the values for a and 7 are wanted, determine 

 the values for a very large number of fragments; the minimum 

 value will be a and the maximum 7. 



Determination of the Refractive Index of a Liquid by the 

 Method of the Displacement of Images. When an object is 

 viewed through a liquid from a point in a line normal to the plane 

 in which the object lies, the image observed will appear to lie in 

 a plane above that of the object, the amount of displacement being 

 dependent upon the refractive index of the interposed medium. 1 



FIG. no. 



If, therefore, we place a liquid in a cell of depth DD' (Fig. no) 

 and measure the amount of displacement of image OO' of a mark 

 at O upon the lower surface of the glass slide, the index of re- 



DD' 

 fraction n will be found from the equation n 



1 This method is very old and is generally known as the Due de Chaulnes 

 Method, having been described by him in 1767-1770. 



See also Sorby, Chem. N., 37 (1878), 151; Watson, Physics; Johannsen, Manual 

 of Petrographic Methods. 



