238 



ELEMENTARY CHEMICAL MICROSCOPY 



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 



If, therefore, we place a liquid in a cell of depth DD' (Fig. 140) 





-o' 



FIG. 140. 



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

 mark at O upon the upper surface of the glass slide, the index of 



DD' 



refraction n will be found from the equation n = - 



Method 1. A Cell and Cover Glass of Known Thickness. 

 Cement upon an object slide of clear glass a cell whose top and 

 bottom are ground true and parallel. After the cement has 

 hardened, determine the depth of the cell by means of calipers, 

 dial gauge or by means of the micrometer screw of the fine 



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. 



