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then for two reasons. As we will show later how 

 to focus an objective, it may be of value to the 

 student to take no risk in endangering- the object 

 and in the case of oil immersion objectives, where 

 the working- distance is exceedingly small, to know 

 what thickness of cover glass may be used. In 

 instruments having a graduated micrometer screw, 

 bring the front of the objective just in contact 

 with the top of the cover glass. This can best be 

 done with no danger to either objective or cover 

 glass, by grasping the instrument beneath the 

 base and raising it so that the cover glass is level 

 with the eye. By looking toward a window the 

 slightest space can be seen. In the case of a heavy 

 instrument lower the head to the level of cover 

 glass. Note the division and by an upward focus- 

 ing turn of the screw, bring the object in focus and 

 read off the distance traversed. In the case of an 

 oil immersion objective follow the same process by 

 bringing the objective in contact with the cover 

 glass dry, then separate slightly and inclining the 

 instrument allow some of the oil to drop into the 

 space, then focus and read off. 



In the case of a micromenter screw without 

 graduations the matter becomes more difficult, but 

 can be done in the same manner although not so 

 accurately, by marking the milled edge of the head 

 of screw with a wax pencil or ink, in the two posi- 

 tions, by taking the middle of the arm as a fixed 



