When the circle of light has passed beyond the 

 limit of aperture of the objective the field will become 

 dark. The amount of illumination may be modified, 

 but in a general way it may be said that the best 

 results with oblique illumination are obtained by 

 reducing the amount of illumination to its minimum 

 practicable amount. 



In objects with striated structure, the illuminating 

 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 plate. 



In using process II the circle of light should be 

 bright. As it nears the edge, colors become apparent 

 until finally at the edge of the objective the violet 

 and red are quite pronounced. With proper disposi- 

 tion of the mirror the colors may be so equalized 

 that after a little practice the illumination will be found 

 at its best after the eyepiece is applied. 



With either method lamplight will be found to 

 give best results but care must be taken that, as the 

 diaphragm passes toward the oblique point, the mirror 

 is also turned so that the illumination will not be lost. 



In the Abbe condensers the chromatic aberrations 

 are quite apparent with extreme oblique illumination 

 more so with that of 1.40 N. A. than with 1.20 N. A. 

 The field ceases to be equally illuminated and all the 

 colors of the spectrum from the violet to the red are 



165 



