HIGH-FOWER AND ROUTINE MICROSCOPY 187 



13. Use a Nelson Cassegrain or similar condenser for 

 dark field with objects in immersion oil or balsam, with 

 high-power oil-immersion objectives; and a cardioid or 

 bicentric condenser for objects in water, with oil-immersion 

 objectives of lower aperture; both with a centering ring in 

 the substage sleeve. 



14. Have the slides 1.0 millimeter thick, within 0.1 

 millimeter. This is important. 



15. Have the covers 0.17 millimeter thick (or 0.01 less), 

 whenever fine details are to be observed. This is also 

 important. 



16. Use a water-immersion objective (with a correction 

 collar) for objects in water, except for those close to (within 

 10 microns of) the cover-glass. 



17. Have the objectives well centered on the nosepiece 

 (and the condenser permanently centered with the high 

 oil-immersion objective), 



18. Omit the high dry objective, unless it has a correction 

 collar. 



19. Have no objective on the standard microscope lower 

 than 8 or 10 times. 



20. A fluorite oil-immersion objective, used with yellow- 

 green glass may be nearly equal to an apochromatic objec- 

 tive, for observation. 



21. Only apochromatic objectives are to be employed for 

 photography with blue light. 



22. Keep the objectives, especially, optically clean. 



23. See that the nosepiece has no backlash, and is 

 rotated the right way. It should be carefully centered by 

 the maker. 



24. Keep the mechanical tube length normally at the 

 standard length. 



25. Even with oil-immersion objectives, increase the 

 tube length slightly when the cover-glass is thinner than 

 normal. For a 90 oil-immersion objective, this increase 

 may be about 1 millimeter for each 0.01 millimeter below 

 normal. 



