46 



The draw-tube which is possessed by all stands, with the exception of 

 Stands IX and X and the Hand -microscope, admits of the tube -length (see p. 3) 

 being increased or diminished. With Stands I to IV the length of the tube may 

 be read off on the scale engraved upon it. The lower end is provided with 

 the standard screw to take an objective of lower power, as, for instance, the 

 auxiliary objective used with the apertometer. 



The internal diameter of our draw-tube is 23.3 mm at the eye-piece 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 focusing 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 focusing and centring with 

 much greater exactness 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. 



Respecting the elongation of the tube produced by the adaptation of these 

 appliances see the remarks on page j. 



1. Revolving nose-pieces, Nos. 48 50, Fig. 20. 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 posi- 

 tion being indicated by the snapping of a spring. 



