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. f 
If the condenser is not provided with an engraved circle upon i 
its upper surface it may be centered by placing an object slide ; 
upon the stage with immersion fluid, usually oil, between it i 
and the condenser; the light spot from the radiant is next prop- j| 
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 |i 
lie to one side, bring it to the center by means of the centering I 
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 I 
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 
