24 
ELEMENTARY CHEMICAL MICROSCOPY 
an objective of low numerical aperture. The iris diaphragm 
is shown well closed. Usually it is advisable to also lower the 
condenser. Failure to employ the Abbe condenser in the proper 
manner or to appreciate the fact that a different adjustment is 
required to meet different problems, is doubtless responsible 
for more errors in interpretation in microscopic examinations 
than any cause other than excessive magnification. Since very 
few dry achromatic objectives 
have a high numerical aperture 
it is evident that in order to 
obtain the best results it will 
be essential with all such optical 
combinations to close the iris 
diaphragm of the Abbe condenser 
until the numerical aperture is 
no greater than that of the ob¬ 
jective. It will be found to be 
a safe general rule to lower the 
Abbe condenser and to close its 
iris diaphragm to a diameter 
about two-thirds or one-half 
that of the rear lens opening of 
the objective. The size of the 
diaphragm opening may easily 
be adjusted by removing the 
ocular, looking into the tube of 
the microscope and closing the diaphragm until the bright disk 
of light is reduced one-half or two-thirds. 
Oblique illumination with the Abbe condenser is quickest 
and most easily obtained by the method suggested by Wright 
of holding a finger below and half across the opening of the 
condenser; the light rays then take the path roughly indicated 
Fig- 5- Or we may drop upon the swing-out ring attached 
to the bottom of the condenser mounting a half-disk of black 
paper or cardboard, or a disk provided with a circular opening 
to one side of the center. The disks furnished with the conden¬ 
ser, consisting of a central stop with narrow slots, yield very 
Oblique Light. 
