5 i8 GENERAL BIOLOGY 



objectives by turning the nose piece, and focus, observing 

 whether the lens must go up or down, and how many turns 

 of the fine adjustment screw. If the two objectives are 

 exactly concentered, nothing further will be necessary. If 

 not, an object placed in the centre of the field with the low 

 power may not be in the field at all with the high. To find 

 where it is reverse the process. Pick out some easily recog- 

 nizable part seen under high power; change objectives 

 again, and see where it lies in the low power field. Here 

 will be the place to put an object, in order to find it again 

 under high power. Always begin hunting it with the low 

 power. If your objectives do not happen to be concen- 

 tered, the above process may have to be repeated every 

 time an objective is loosened and its position changed. 

 Practice finding things with high power until you can do it 

 quickly and certainly. 



Practical points in the use of miscroscope and of stage 



mounts. 



1. Save your eyes, by proper use of them. First focus; 

 then look. 



2. Look through the centre of a lens, not through its 

 edges. 



3 . Look with both eyes open, concentrating attention on 

 what is before the one, disregarding what is before the 

 other; this will save much weariness of many facial muscles, 

 when once an acquired habit. 



4. Never rack the microscope tube downward while 

 looking through it. 



5. Always use a coverglass with a high power objective. 



6. Wet objects should be completely immersed for 

 examination. 



7. A cover glass has several mechanical uses: a) 

 Mounted objects of some kinds (such, for example, as fully 



