26 
ELEMENTARY CHEMICAL MICROSCOPY 
diaphragm opening. The diaphragm opening should fall at the 
center of the field of view directly under the cross-hairs, con¬ 
centric with their point of intersection. If the image of the 
opening is not centrally located there is something faulty in the 
construction of the condenser or in its attachment to the sub¬ 
stage, or in the alignment of objective and ocular. 
If the condenser has been found centered, we may change to 
a high-power objective and be reasonably sure that the con¬ 
denser will be centered with respect to the objective, providing a 
revolving nose-piece is not in use; but if the objective is attached 
to an ordinary nose-piece, turning from one objective to another 
usually necessitates a readjustment of the condenser. With high 
powers, centering, as described above, is impossible and it will 
be found simpler to remove the ocular and hold a tripod or 
pocket magnifier over the tube; the image of the diaphragm open¬ 
ing is then easily seen and Its relative position ascertained. 
In testing for proper centering it is important that the mirror 
be so placed as to yield exactly axial light. This may be assured 
by swinging the condenser to one side and placing upon the 
stage a preparation consisting of thin gum beaten up until full 
of air bubbles; a very tiny air bubble is selected and brought 
to the center of the field, it appears as a bright spot surrounded 
by a black ring (see page 229); the bubble is sharply focused 
and the mirror adjusted by proper tipping until the bright spot 
appears exactly at the center of the circular black ring. The 
light is now exactly axial. This method of assuring absolutely 
axial light ^ is the simplest and surest available. 
Without touching the preparation or the mirror, carefully 
swing the condenser back in place, raise it about halfway and 
slowly raise and lower the body tube by means of the coarse 
adjustment, closely observing at the same time the appearance 
of the bubble image. If the light still remains axial with the 
condenser in place there will be no appreciable swaying of the 
image and no change of position of the bright spot of light. If 
the image sways and the bright spot of light is displaced to one 
side of the center the Abbe condenser is faulty and the character 
^ Gage, The Microscope, p. 48, loth Ed., Ithaca, 1908. 
