43 



and Lens Holder L II, all the other stands are fitted with rack and pinion ad- 

 justment. We have constructed special machines for the accurate production 

 of the oblique toothgearing, and this motion is now made in such perfection that 

 even objectives of medium power can be focused with sufficient exactness by it 

 alone without the use of the micrometer-screw. 



The fine adjustment. The micrometer movement introduced by us in 1886 

 (see Zeitschr. f. wiss. Mikroskopie, vol. Ill, 1886, pp. 207 209) has during this 

 long period been in extensive use and has proved absolutely satisfactory. We 

 have therefore retained it in most of our stands for the present. The superiority of 

 this arrangement is mainly due to the fact that the force exercised by the micro- 

 meter-screw is transferred to the movable body by a single contact between 

 two hardened steel surfaces, which ensures an extremely delicate and uniform 

 motion. 



In 1898 we introduced an entirely new type of construction 

 of the upper part of the Stand. The coarse adjustment by means 

 of rack and pinion was retained, but the mechanism for fine adjust- 

 ment underwent a complete change. 



The then generally used prismatic movement was discarded in favour of 

 one by means of slides. The sliding piece itself is situated close behind the 

 coarse adjustment action, with which it is rigidly connected by screws. It bears 

 on its lower half an elongated nut for the micrometer-screw and above that a 

 worm-spring the direction of whose pressure lies in the prolongation of the axis 

 of the micrometer-screw (see Fig. 19). 



A worm-wheel, movable by means of an endless screw, is situated at the 

 lower end of the micrometer-screw and serves in a sense as a screw-head. 



. Here also the force exercised by the micrometer-screw is transferred to the 

 movable body which carries the tube by a single contact between two hardened 

 steel surfaces. 



The whole mechanism, with the exception of the two milled-heads acting 

 upon the endless screw, is enclosed inside the crane-shaped carrier and 

 is thus protected against all direct damage from outside. An additional safety 

 appliance prevents injury to the micrometer-screw when the sliding piece has 

 reached the limits of its movement. 



As the axes of the milled-heads used in the fine and coarse adjustment are 

 parallel, and as in elevating and depressing the tube both turn in the same 

 direction, the change from one movement to the other is most convenient for 

 the hand of the operator. 



