43 



is only absent in the inner tube of the large mineralogical Stand III M ; instead 

 of a screw thread a slide is made use of, into which the BERTRAND Lens can 

 be inserted from the outside. (See pp. 50 and 51.) 



The internal diameter of our draw-tube is 23.3 mm at the ocular end. 



D. Appliances for Changing the 

 Objectives on the Stand. 



Revolving Nose-pieces or Sliding Objective -changers are appliances 

 designed to facilitate the rapid and commodious interchange, while interfering 

 as little as possible with the focussing and centring, of the objectives. 



The sliding objective changers are in so far superior to the revolving nose- 

 pieces that they admit of the maintenance of the focussing and centring with 

 much greater precision than the latter are ever capable of giving. Another 

 advantage is that in using them the number of objectives is not restricted to 

 any particular limit. 



In using nose-piece or objective-changer, the length of the nose- 

 piece (15 mm) and that of the objective-changer (22 mm) should 

 be taken into consideration, and the draw-tube shortened by the like 

 amount. (See p. 8.) 



Nos. 48 50. Revolving Nose-pieces, Fig. 18. "A clamping ring with 

 milled edge serves to fix the nose-piece in any desired position -- as a rule so 

 as to have the objectives not in actual use turned symmetrically towards the 

 front. The objectives are screwed into the nose-piece and can by a simple turn 

 be placed in rapid succession in alignment with the tube, the correct position 

 being indicated by the snapping of a spring. 



A curved cover, turned by special methods of our own, fits closely over 

 the openings into which the objectives are screwed and protects them against dust. 



