TJie Operation of a Microscope 93 



piece. Then, while watching tlie bottom of the objective from the side, 

 lower the tube initil the objective ahnost touches the cover glass. Look 

 into the microscope and rack up the tube until the specimen is in focus. 

 Remember, when an objective of a different numerical aperture is used, 

 the condenser diaphragm must be adjusted (step 6 above). The numeri- 

 cal aperture (N.A.) number is used to compare the resolving power of 

 the lenses; the higher the numerical aperture, the greater the resolving 

 power and the condenser diaphragm will have to be opened wider. 



Use of Oil-immersion Objective 



The working distance with this objective is short and great care must 

 be taken to prevent crtishing a cover glass. Thin cover glasses mtist be 

 used or the specimen motnited on the cover glass itself (page 385). 

 Cheater illumination is required than with the dry objectives and a 

 wider cone of light is needed. The space between the objective and the 

 cover glass, also frequently bet^veen the slide and the condenser, must 

 be filled with a stiitable mediinn of a refractive index and dispersion 

 like that of the glass of the lenses. This mediimi is provided by immer- 

 sion oil (cedarwood oil, cro^vn oil). 



1. Using a dry objective locate and center the area to be examined. 



2. Raise microscope tube, swing oil-immersion objective into position. 



3. Place a drop of immersion oil on the cover glass over the area to be ex- 

 amined. Watching (from the side) the lower edge of the objective, lower 

 it slowly imtil it makes contact with the oil. 



4. Observing through the microscope, very slowly lower the objective with 

 the coarse adjustment until the specimen is in focus. Make more critical 

 focus with the fine adjustment. 



5. Remove ocular eyepiece and adjust the light on the back lens of the objec- 

 tive; the condenser diaphragm will have to be opened. 



After use, clean oil-immersion objective; never leave oil on it. Wipe it 

 with clean lens paper, then with a little xylene or chloroform on lens paper, 

 finally drying it with clean dry lens paper. Do not leave xylene or chloroform 

 on the lens. 



Other Hints for Efficient Microscopy 



Always keep oil siufaces clean, free from dust and grease. This includes 

 the lamp condenser, filters, mirror, lenses of the substage condenser, 

 lenses of objectives, slides, cover glasses, binocular prisms, lens of 



