MANIPULATION OF THE MICROSCOPE. 27 
objects, oblique illumination brings out certain structures 
best. Light so oblique that it cannot enter the objective 
at all is sometimes obtained by placing under the stage 
a stop cutting off the central cone of light. In such a 
preparation the object will be made visible by the rays 
which it reflects or refracts upward, and will appear 
self-luminous on a black background—dark-ground 
illumination. 
With objectives higher than } in. even the concave 
mirror will not give sufficient light for successful micro- 
scopic work, and the Abbé condenser must be used, with 
the plane mirror. The condenser, as has been stated in 
Chapter I, is an objective system of lenses placed just 
under the stage. in such a position as to concentrate a 
considerable amount of light upon the object to be ex- 
amined (Fig. 19). With low powers the condenser should. 
be swung out from its place to one side; while with the 
7z-in. immersion objective the best results may be obtained 
by placing a drop of oil between the condenser and the 
bottom of the slide, making the entire system optically 
homogeneous. When one is working with these high 
magnifications it is also important that the condenser 
should be accurately centered and focussed; that is, it 
must be at such a distance below the stage that the great- 
est possible amount of light may be concentrated upon 
the object. This position may be determined by focus- 
ing upon some specimen with a 3-in. objective and then 
so adjusting the condenser that the image of a window- 
sash or of a flame coincides with the object itself. 
It has been pointed out that the function of the dia- 
