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 



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° 



