Appliances for changing 
the objectives on the stand. 
These contrivances are required to satisfy two conditions : 
1) tliat the image should not disappear on changing the objectives, 
so that only a touch of the micrometer screw is necessary for 
perfect adjustment, and 2) that the centering should be good, 
i. e. that the same spot in the specimen remains in the field after 
changing the lens. 
The first condition is satisfied by adjusting the lengths of 
the funnels of the objectives which are required to be inter- 
changeable in such a manner that on changing them their foci 
may lie at a corresponding distance from the plane of the object^). 
The second condition can only be realised in the ordinary "re- 
volving" nose-piece if this is specially adjusted for the particular 
objectives intended for use with it. It is impossible, therefore, 
to be answerable for the exact centering of nose-pieces sub- 
sequently supplied. To avoid this inconvenience we have lately 
constructed the Sliding Obj ective- changer (see below) 
which possesses special means for centering and can, therefore, 
be adapted to any objective. 
*) This applies to the achromatic lenses A to F on the one hand , the 
apochromatic lenses 16-mm to 3-mm (ap. 0.95) on the other. Lens a^ cannot be 
used with any objective-changer; objectives ag, ag, a* and aa can be used with 
objective-changers on those of our stands which are fitted with rack and pinion 
adjustment. 
