QUANTITATIVE ANALYSIS BY MEANS OF THE MICROSCOPE 217 
It is absolutely essential that the plane mirror of each micro¬ 
scope shall reflect light of equal intensity. The adjustment 
must therefore be made in advance. The colorimeter tubes 
are filled with distilled water and placed one on each of the 
stages of the microscopes. Each microscope is then focused 
in turn upon the surface of the liquid in the tube and the tubes 
moved until centered with respect to the optic axis of the micro¬ 
scope. The microscope mirrors are now tipped back and forth 
until the two halves of the eyepiece field are of equal intensity. 
Not infrequently it will be found necessary to take the light 
for the mirrors from a large square of ground glass placed in a 
window or from a sheet of pure white paper similarly placed. 
It is essential that the final depths of the liquids under com¬ 
parison shall not be far apart, since absorption of light as well 
as color intensity must be taken into account. 
A very sensitive assay or a Nernst “ Micro ” balance must 
be employed for weighing the unknown materials.^ 
2 See also on this method of analysis: 
Emich and Donau: Monats. Ch. 28 (1907), 826. 
Donau: Monats. Ch. 36 (1915), 381- 
Emich: Monats. Ch. 36 (1915), 407-440. 
Donau: Die Arbeitsmethoden der Mikrochemie: Stuttgart, 1913. 
