ILLUMINATION OF OBJECTS; DARK FIELD 
43 
optic axis of the microscope; (5) accurate centering of the objec¬ 
tive.^ 
An examination of the diagrams (Figs. 14 and 19) will show 
that theoretically the oblique rays meet to form a tiny spot of 
light just outside the apparatus in the line of its optic axis. It 
is obvious that this spot should lie in the optic axis of the objec¬ 
tive and the ocular. In order to facilitate centering, a tiny circle 
is usually engraved upon the upper surface of the glass of the 
illuminator; this circle is focused with a low power and is brought 
to the center of the field of the microscope, by means of center¬ 
ing screws c, c, Fig. 20, provided for this purpose. 
When working with the Bausch & Lomb “ Dark-ground 
Illuminator ” shown in Fig. 20, care should be taken to start 
observations with the 
diaphragm d, opened to 
its full aperture. If the 
preparation fails to yield 
a satisfactory dark field 
the diaphragm should 
be slowly closed until 
the best results are ob- 20. Paraboloid Dark-field Illuminator. 
tained; a darker field 
and brighter particles will probably result, but the resolution 
will be somewhat poorer. If, however, the diaphragm be closed 
too far, as, for example, as shown in the right half of Fig. 20, 
no light can enter the annular opening in the paraboloid and 
the apparatus will fail to function. 
If the microscope is pro\dded with a revolving nose-piece 
the objective used in centering should be removed and the high 
power to be employed in the dark-field studies substituted in 
the same opening in order that there shall be no change in the 
relations of the optic axes. When employing 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 objec- 
^The adjustment and method of use of Dark-field Illuminators is discussed in 
great detail by Gage. Trans. Amer. Micro. Soc. 39 (1920) 95. Workers with 
dark-field illuminators should not fail to consult this paper. 
