ILLUMINATION OF OBJECTS; ILLUMINATING DEVICES 47 



a revolving nose-piece the objective used in centering should be 

 removed and the high power to be employed in the dark-ground 

 studies substituted in the same opening in order that there shall 

 be no change in the relations of the optic axes. When employ- 

 ing ultracondensers of the highest type it is better to remove 

 the nose-piece and to attach to the body tube a centering adapter 

 into which the objective is screwed; this permits accurate cen- 

 tering of each objective used and therefore much better optical 

 conditions are obtainable. 



d 

 FIG. 22. Paraboloid Dark-ground Illuminator for Use below the Stage. 



In order, however, that the objective may be centered, it is 

 essential that we have a central fixed point upon the stage to 

 which we may refer. Stands to be employed for high-grade 

 ultramicroscopic work should be provided with mechanical 

 stages with graduated coordinate motion and a centering object 

 slide, carrying at its center a tiny cross. When placed upon 

 the stage so that the different scales of the mechanical stage 

 occupy the positions which the manufacturer has indicated upon 

 the object slide, the point of intersection of the ruled cross will 

 fall exactly in the axis of the tube of the microscope. The 

 objective is focused sharply upon the cross and if the center of 

 the cross does not fall in the center of the field it is brought there 

 by moving the screws a, a, Fig. 24, page 59. 



If the condenser is not provided with an engraved circle upon 

 its upper surface it may be centered by placing an object slide 

 upon the stage with immersion fluid, usually water, between it 

 and the condenser; the light spot from the radiant is next prop- 

 erly adjusted and the mirror inclined until a bright spot of light 

 appears upon the object slide. The condenser is raised or lowered 



