44 ELEMENTARY CHEMICAL MICROSCOPY 



tive is screwed ; this permits accurate centering of each objective 

 used and therefore much better optical conditions are obtainable. 



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 objec- 

 rive 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. 51, page no. 



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 oil, 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 

 until the spot of light attains its smallest size. Focus upon this 

 tiny spot with a low-power objective; if the condenser is properly 

 centered the spot will lie at the center of the field. Should it 

 lie to one side, bring it to the center by means of the centering 

 screws or center the objective with respect to the point of light. 



Having adjusted the condenser, the next step, if the device 

 is of the cardioid type (see page 117), is to ascertain whether 

 the quartz cell, which must be used with the instrument, is in 

 proper condition for use. Lay the quartz cover upon the cell 

 and press it down very carefully. Notice whether there appears 

 at the zone of contact between cell and cover a series of colored 

 concentric rings. If the pattern does not consist of concentric 

 circles, but appears to be elliptical, it is probable that the cell is 

 not level with respect to the optic axis. Adjust the level screws 

 until the plane of the cell is normal to the optic axis. If the 

 eccentricity of the rings does not disappear, the trouble lies in 



