ADJUSTING THE MICROSCOPE 277 



are easily avoided with a little thoughtful attention. One can 

 soon get into the habit of not touching the lenses with the fingers, 

 and reagents will not run out from under the coverglass unless 

 niore than is necessary has been used. One should soon learn 

 how large a drop of reagent is needed for a given size of 

 coverglass. 



For bacteriological and cytological work an oil immersion 

 objective will be necessary. A tV in. or a 2 mm. objective 

 is best for general work in these lines. If such an objective 

 is to be used it would be best to have a triple revolving nose- 

 piece to carry the medium, the high, and the immersion object- 

 ive all at once. The use of the oil immersion objective is 

 very simple. Put a small drop of cedar immersion oil on the 

 coverglass directly over the object and run the body tube down 

 with the coarse adjustment until the front lens of the immersion 

 objective enters the drop and comes almost into contact with 

 the coverglass. This is to be done while watching the objective 

 outside the microscope. Then while looking through the ocular 

 draw the objective up with the fine adjustment until the object 

 comes into focus. 



The higher the power of the objective the smaller the field 

 of view, and when it comes to the oil immersions one sees but 

 a very small area at once, and it is therefore much more diffi- 

 cult to find a small object with them than with the lower powers. 

 For this reason it is a good plan to find the object with a lower 

 power and bring it to the center of the field, and then it 

 will probably be in view when the immersion objective is used, 

 but if it is not it cannot be far out of the way and a very slight 

 movement of the preparation one way and another should 

 suffice. With the oil immersion, when the object has 

 been stained, the opening of the diaphragm may be made as 

 wide as is necessary to let in plenty of light. Indeed a good 

 image can under these conditions be secured with a wide open 

 diaphragm. When through with the oil immersion objective 

 wipe it ofif with a clean soft cloth or with a piece of Japanese 

 lens paper, and wipe off the coverglass in the same way. 



i 



