106 



center up to the limit of aperture of the objective. 

 When beyond this, the object will appear illuminated 

 on a dark ground. The lateral movement should be 

 at right angles to the striae which it is desired to see, 

 and if this is not known, either the object should be 

 revolved or the mounting of the condenser. It will 

 sometimes be found advantageous to reduce or 

 enlarge the opening. 



