162 



lens of objective and then moving this opening 

 laterally across the bottom of condenser to such a 

 point as will bring out the structure of the object, 

 or to the limit of the aperture of the objective. 

 When beyond the limit of aperture the field will 

 appear to become dark. The amount of illumina- 

 tion may be modified, but in a general way it may 

 be said that the best results with oblique illumina- 

 tion are obtained by reducing the amount of 

 illumination to its minimum practicable amount. 

 In objects with striated structure, the illuminat- 

 ing rays should be brought to a position at right 

 angles to the striae, either by rotating the object to 

 the proper position, or by swinging the diaphragm. 



