360 OSCAR W. RICHARDS 



insert a stop near the back lens of the objective to decrease its aper- 

 ture to match that of the condenser. 



Parallel light should be used and the condenser must be carefully 

 centered for uniform illumination. The slides must be very clean 

 and free from scratches and imperfections and of the thickness for 

 which the condenser is designed. Colored or stained tissues may be 

 examined. Particles smaller than the limit of resolution may be 

 detected when the light is strong enough, although their shape and 

 size cannot be measured. In fact, the smallness of the particles that 

 may be detected depends only on the intensity of the light. Very 

 strong illuminants should be used to make the energy in the center 

 of the diffraction disc (see Fig. 1) great enough to stimulate the retina 

 and be seen. Some information as to the number of particles and 

 their average size may be inferred when the depth of the specimen and 

 area examined are known. The darkfield microscope is useful for 

 locating the spirochetes of syphilis in exudates from sores and in the 

 study of small cellular inclusions and the larger colloidal materials. 

 Microscopes with built-in, precentered darkfield condensers are avail- 

 able {57) and most brightfield microscopes can be adapted for dark- 

 field microscopy by substituting a darkfield condenser in place of the 

 brightfield condenser. Some practice in centering the equipment and 

 interpreting the image is required, but the method is useful and not 

 particularly difficult (4). 



2. Ultra microscopy 



The ultramicroscope passes a narrow beam of intense light through 

 the specimen from one side and the observer sees the specimen by its 

 Tyndall effect, as with particles in a beam of sunlight. It is useful 

 for the study of smokes and colloids and requires special equipment 

 and considerable skill for efficient use (3). 



3. Optical Staining 



When the central part of the condenser stop is made of a trans- 

 parent colored material (rather than opaque as in darkfield stops) 

 and the annulus surrounding it is made of a complementary colored 

 material (Fig. 7B), reasonably transparent specimens will appear as if 

 stained by the color of the annulus against a background of the color 

 of the center stop. Crystals, inclusions, and fairly transparent mi- 

 croscopic plants and animals may be observed in this manner. Col- 



