74 



PHOTO-MICROGRAPHY 



another in. The portion separately attached to each objective is supplied with two 

 adjusting screws, which enable the microscopist to move the objective from side to 

 side or from above downwards. This adjustment is done once for all, provided the 

 objective be not removed from the holder, when perhaps readjustment may be 

 necessary. The convenience of such an appliance must be experienced to be 

 appreciated, but a word of caution is necessary in centreing objectives with these 

 changers, and it is this : Seeing that each objective can be moved from side to side or 

 from above downwards in its holder to a sensible amount, it is quite possible so to 

 shift the objective that the object is unconsciously viewed almost entirely through its 



outer zones only, instead of along its optical axis. The 

 ^^s- 40 consequence of this is that the perfection of definition 



may be seriously interfered with ; just as if with an 

 ]Yl\ytY^N/|\^ Ln ordniary photographic lens and camera a photograph was 

 pJ taken with the lens pushed up or down in the sliding 

 ^Ji^J^^^^ ^ front, and the photograph taken with the marginal rays 

 , I— T _L instead of the central ones (Fig. 40). The way to avoid 



Camera with lens pushed up this mistake is to adopt the following plan. Let us 



suppose the microscopist is desirous of centreing several 

 objectives for the same microscope. Having secured a sensibly sized diatom- — one 

 preferably with a well-defined edge and centre, such as one of the Arachnoidiscus 

 type — let it be placed on the stage of the microscope, and a ^ inch objective (as it 

 comes from the maker) screwed into the nose-piece of the microscope tube in the 

 ordinary fashion. The diatom is then fidgetted about on the stage until it is practi- 

 cally centra] in the field of view. 



This power is then removed and the highest objective of the battery placed in its 

 stead, say, for instance, a xVth immersion. Having again focussed the diatom, its 

 central position is brought accurately into the middle of the field and the objective 

 removed. The " body portion of the changer " is now affixed to the microscope tube 

 and the other portion of it to the xVth, and the two united by sliding them together. 

 The diatom by our previous experiment was placed central with the axis of the tube, 

 and it only now remains to see whether it is still so placed. If not, it must be centred 

 by the adjusting screws on that portion of the changer attached to the objective, and 

 not hy moving the diatom. Each objective must be centred by its own changer in the 

 same way the diatom remaining untouched throughout. To make our meaning clear, 

 the first portion of the description deals with the method of getting the diatom in the 

 axis of the tube of the microscope, the second shows how to make the axis of each 



