66 GENERAL MICROBIOLOGY 



sity of screwing the different objectives in as needed and 

 to protect the back lens of the objective from dust. The 

 later microscopes have a " collar " nose-piece which keeps 

 the objectives free from dust at all times. 



1. Nose-pieces and objectives of the best makes are now 

 made so that the objectives are parfocal, i.e., when one lens 

 is in. focus the others on the nose-piece will be nearly in 

 focus when they are swung into the optical axis. They 

 are also approximately centered so that a point in the 

 center of the field of one lens will be in the field of the 

 others. 



2. Objectives made parfocal for one tube-length or eye- 

 piece are not parfocal for a different length or a different 

 eye-piece. 



3. Objectives of one microscope should not be inter- 

 changed with those of another, even if of the same make. 



4. Always focus up, slightly, before turning from a lower 

 to a higher power. Otherwise the front of the objective 

 may be swung against the cover-glass and injure both the 

 specimen and the objective. 



The Optical Parts. The optical parts are the lenses of 

 the objectives, oculars and condenser and the mirror. 



1. Wipe dirty lenses gently with Japanese lens paper to 

 remove dirt. 



2. Never rub a lens vigorously with anything. 



3. Avoid touching the surface of a lens with the fingers. 

 Cutaneous secretions are hard to remove. 



4. Always clean the oil immersion objective with lens 

 paper immediately after using. If the oil is allowed to dry, 

 xylol must be used to clean the lens. 



5. Always leave an ocular in the tube to keep dust from 

 settling on the back lens of the objective. Dust on the 

 back lens may be removed with a earners hair brush. 



6. Never take an objective apart. 



7. Oculars, condenser and mirror should be kept clean 

 by the use of lens paper. 



