98 THE USE OF THE MICROSCOPE 



with apochromatic or fluorite objectives, and a "daylight" 

 screen. 



4. It is, in the writer's experience, advantageous to correct 

 the high-power condenser for water immersion, and always to use 

 it immersed, 



5. A dry achromatic condenser is good for routine work. 



6. The corrected condenser should be adjusted, by the maker 

 or the user, for the distance of the lamp, and for the thickness of 

 the slides regularly used. 



7. The corrected condenser should be accurately focused on 

 the object for high powers, and approximately for low powers. 



8. The aperture of the condenser must be carefully regulated 

 (for best vision), regardless of the light intensity, which is to be 

 adjusted by suitable screens. 



9. An aplanatic-achromatic immersion condenser, with center 

 stops, gives a good dark field with the 20 apochromatic objective 

 of 0.65 aperture, or with the dry or oil-immersion objectives of 

 0.85 aperture. 



10. For objective apertures between 0.85 and 1.05 (or even 

 higher), the bicentric or the cardioid condensers show objects in 

 water well, with the special oil-immersion objectives. 



11. For stained smears of bacteria or spirochsetes, or other fine 

 particles, mounted in balsam or immersion oil, the Cassegrain or 

 the Leuchtbild condenser will give good pictures with objectives 

 of 1.3 aperture. 



12. With high objectives, the use of the uncorrected condenser 

 usually entails the use of 5-times eyepieces. With a corrected 

 and adjusted dry condenser, giving 0.9 true aperture, the 10- 

 times eyepieces may well be regularly employed, and the 5-times 

 not used. This is with the usual achromatic objectives of 2 

 or 1.8 millimeters focal length. With apochromatic or fluorite 

 objectives of 3 or 2.5 millimeters focal length, 15- or 20-times 

 eyepieces may be regularly employed, and the 10-times eyepieces 

 not used; there being a corrected and adjusted water-immersion 

 condenser, giving 1.25 true aperture. 



13. If the condenser is not centered with the objective and 

 eyepiece, the margin of the 3-millimeter diaphragm will not seem 

 uniformly sharp all around, nor uniformly colored when out 

 of focus. 



