58 



for certain appearances, it is the only reliable 

 means of covering each portion of it. This stage 

 is especially valuable for blood counting and 

 plankton work. While not many years ago it was 

 spurned as a toy by many scientists, it is now 

 generally accepted as an invaluable part of a 

 microscope. In order to be so however, it must be 

 of the most perfect workmanship, which is difficult 

 to attain on account of the necessarily small parts 

 of which it is composed and the hard usage which 

 it must bear. The movement must be smooth and 

 easy and on reversing the milled heads, must not 

 show any lost motion or dead point. 



There is a variety of forms, but two principal 

 types, one of which is built onto the stage, usually 

 the revolving one, the other attachable (Fig. 23) 

 which may be removed when not wanted, leaving 

 the ordinary stage intact. They are made with 

 graduations, usually divisions in millimeters, by 

 which one may read off the amount of space which 

 is covered. 



In using the mechanical stage it should first be 

 determined how many spaces, or how much of one 

 space is contained within the limits of the field ; 

 then begin at one edge of the specimen and with 

 lateral movement make the object pass across 

 the field. Move the object forward with the other 

 movement the amount of space which has been 

 previously determined and by a return motion 



