MEDIUM-POWER PHOTO-MICROGEAPHY 73 



(iii) The Movable Nose-piece for Rapidly Changing the Objectives.— 



The ordinary method is that of affixing to the end of the tube what is commonly 

 called a revolving nose-piece. These are made to hold two, three or four objectives, 

 and if they could be made so true that when an object is found with a half inch it 

 would be equally in the centre of the field when the eighth is brought into position it 



Fig. 39 



Zeiss Sliding Objective Changer 



would be satisfactory ; but this it seems impossible to do, inasmuch as each objective 

 always requires an adjustment peculiar to itself. It is to meet this difficulty the firm 

 of Zeiss have introduced what they term their "Objective Changer" (Fig. 39). It 

 consists of two parts, one attached to the microscope itself, which is never removed, 

 and another which is fixed to every objective. 



When a change is desired it is only necessary to slide one objective out and 



K 



