MIRROR, STAGE, NUSERIECE, AMJ DRAWTUBE 1:^9 



iVl 



traverser, preparations under medium or large cover- 

 glasses can be systematically searched through; which is 

 not easily done if there are only spring clips on the stage. 

 The writer has used the Detto sliding bar on the round 

 stage for several years in place of a screw mechanical 

 stage, and it gives satisfaction if kept free from balsam or 

 immersion oil. When the condenser is water immersed, 

 the glass slides can be "floated" on a thin layer of water on 

 the stage, so as to run without friction, and this cannot 

 usually be done with a screw ''object traverser." A 

 detachable mechanical stage, or ''object traverser," how- 

 ever, is of use to most workers, especially for the slight 

 movements necessary to get an object in the right position. 

 Many makes are available, and improvements have been 

 suggested in the Zeitschrift fuer wissenschafiliche Mikro- 

 skopie for 1926. The up-and-down motion should be 

 kept tighter than the to-and-fro motion (Zeiss). An 

 important use of the mechanical stage is to act as a finder, 

 by enabhng the latitude and longitude of particular points 

 on the shde to be found and recorded. But this may be 

 done by simpler methods (see Chap. X, page 109). 



In some mechanical stages the movable piece supporting 

 one end of the slide bends in far enough to take a wider 

 slide than 25 millimeters. If only standard slides are used, 

 this piece may be, for ease of working, cut shorter with a 

 triangular file. 



The Nosepiece. — A revolving nosepiece is necessary for 

 the professional microscopist, and the more objectives that 

 can be put on it at once, the better. Four objectives may 

 be needed constantly for some work. The makers of the 

 instrument will center the low and medium objectives on 

 the nosepiece, so that they register fairly accurately with 

 the high objective (see Fig. 10). In this case, one can 

 sometimes do without large centering screws on the con- 

 denser sleeve, presuming that this sleeve also is accurately 

 centered; as it is by the best firms. The writer has one 

 instrument with a large centering condenser sleeve (Baker), 

 and another with a well-centered sleeve (Zeiss) which can 



