GENERAL INTRODUCTION. 3 



tinctly visible do not strain the eye to see 

 . it, but by means of the line adjustment 

 alter the focus of the microscope. In 

 order to bring different parts of an object 

 into view the focus must be continually 

 changed, and for this reason the hand 

 should always be kept on the fine-adjust- 

 ment screw. If this rule be observed 

 there is little danger of injuring the eyes. 

 If the object cannot be brought clearly 

 into view by focussing, see whether the 

 mirror reflects as much light as possible, 

 that the objective is not wet, and that the 

 cover-glass is clean. 



4. The lenses must be perfectly clean. Dust 



on the lens obscures the image, and may 

 strain the eye. To clean a lens, breathe 

 upon the glass in order to dampen it, and 

 then wipe it with a soft, fine cloth kept 

 for this purpose alone. A lens should 

 never be touched with the fingers, as 

 these often leave ineffaceable marks. 



5. Look through the microscope sometimes 



with one eye and sometimes with the 

 other, and keep both eyes open ; the 

 strain upon the eyes is by this means 

 much lessened. 



6. Lenses, when not in actual use, should be 



protected from dust and injury. 



