238 
elemp:ntary 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.^ 
If, therefore, we place a liquid in a cell of depth DD' (Fig. 140) 
Fig. 140. 
and measure the amount of displacement of image 00' 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 = j|^7- 
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 
^ 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. 
