268 THE USE OF THE MICROSCOPE 



49. Comparison of Water- and Oil-immersion Objec- 

 tives. — Take the 70 water-immersion and the 60 oil-immer- 

 sion apochromatic objectives. Focus and adjust them for 

 an object mounted in immersion oil. Note that the oil- 

 immersion objective can focus the object at any ordinary 

 depth, with perfect images; but that the water-immersion 

 objective has to have its collar changed for different depths. 



50. Testing Revolving Nosepiece. — Remove the nose- 

 piece, and screw the high-power, oil-immersion objective 

 directly on the microscope tube. Focus it, without using 

 oil, on a conspicuous object, such as an isolated pollen 

 mother cell, or a small, round or triangular diatom. Get 

 the object in the center of the field, with the circular image 

 of the 2- or S-miUimeter diaphragm around it. Then 

 replace the nosepiece and try the same objective in the 

 different screw sockets of the nosepiece. If it centers 

 perfectly with the object in one of them, the lower objec- 

 tives can be made concentric with it, as already explained. 

 If still out of center, the objectives and nosepiece should be 

 sent to the maker for adjustment. This applies to objec- 

 tives which have not been centered on the nosepiece by 

 the manufacturer. The optical axes of high-power objec- 

 tives may differ by a few hundredths of a millimeter, so 

 that they cannot well be made quite concentric. 



