34 LABORATORY GUIDE IN BACTERIOLOGY 



Be careful not to drop the oculars or objectives. 



6. The incUnation joint can be used only with dry 

 lenses and dry objects, not with the oil-immersion or 

 hanging-drop preparations. 



7. After placing the object on the stage, focus with 

 a low power and the coarse adjustment. With high 

 powers use the coarse adjustment first and the fine adjust- 

 ment afterward. The free use of the fine adjustment 

 saves the accommodation of the eye. As the eye is cap- 

 able of accommodating itself to distances, it may with an 

 effort distinguish a picture which is not in perfect focus. 

 This effort is saved by using the fine adjustment. 



8. Always raise the draw-tube by means of the coarse 

 adjustment before changing the objectives or examining 

 a different preparation. 



9. Before focusing, obtain as good a fight as possible 

 by turning the mirror, and then regulate the supply by 

 the diaphragm. Always use reflected light, never direct 

 sunlight. 



10. When focusing an object, lower the draw-tube 

 until the lens almost touches the cover-glaSs. This can 

 easily be seen by looking at the instrument from one side 

 and watching the reflection of the objective in the cover- 

 glass. Then, with the eye at the ocular, slowly focus up. 

 Do not focus down with the eye at the ocular, as the lens 

 may then come into violent contact with the object, 

 destroying the latter and injuring the lens. 



11. For living or transparent objects use as little 

 light as possible. For stained or opaque objects more 

 fight is necessary. 



12. Do not use higher powers than is necessary. 



13. To use the oil-immersion lens, place a drop of 



