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 J in. even the concave 

 mirror will not give sufficient light for successful micro- 

 scopic work, and the Ahh6 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 | k 

 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 

 i^-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 fdcus- 

 ing upon some specimen with a §-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-- 



