36 METHODS OF PETROGRAPHIC-MICROSCOPIC RESEARCH. 
objective. By examination of this disk C'" D'" with a lens we are able to 
determine at once whether the substage diafram and the condenser lens are 
centered and also whether the upper objective lens is encumbered with dust. 
To be satisfactory the diameter of this eye-circle should not be greater than 
that of the pupil of the eye. In this case the eye receives all the light 
-Retinal image 
J Condenser 
---Diaphragm 
FIG. 30. 
