30 



PHOTOMICROGRAPHY 



Others maintain that the substage ring is the correct 

 place for centring screws, as the errors of centration 

 introduced by a change of objective are not so great as 

 those usually found when the condenser is changed, 

 and they consider that it is best to take the objective 

 as fixed and centre the condenser to it. Either method 

 is perfectly satisfactory when properly employed, but 



Fig. 11 



SLIDING OBJECTIVE CHANGER 



there seems less likelihood of improper use when the 

 centring screws are attached to the condenser ring. 



Centring Nose-pieces and Objective Changers. If the 

 microscope has a fixed substage a centring nose-piece 

 should certainly be added. A simple form consists of a 

 ring with centring screws, which screws into the micro- 

 scope tube, and to which objectives are attached in the 

 ordinary way. 



There are several types of nose-piece designed for 

 the rapid change of objectives. The rotating form shown 



