72 



BACTERIOLOGY. 



freely admits of viliration, and is most inconvenient for laboratory 

 work, The heavy horse-shoe form is compact and firm, and the 

 weight of it can hardly be considered an objection. 



The tubular hod;/ is from eight to ten inches in length, and within 

 it is a, dra,w-tube with engraved scale. By extending the draw-tube 

 greater magnification is obtained ; but as this is at the cost of 

 definition it should hardly ever be used in the examination of 

 bacteria. 



A triple nose-jiiece is a great convenience, saving the time which 

 is otherwise spent in replacing oljjectives of different magnifying 

 power, and there is less risk of injuring them. 



Focus should be oljtained liy means of a rack and pinion coai'se 



Fig. 17.— Removable Mechanical Stage. 



adjustment. The sliding tube is not to be recommended, as the 

 motion may be stiff, encouraging the use of force, which in turn may 

 result in the objective being brought violently into contact with the 

 specimen, injuring the lens or damaging the preparation ; or it may 

 get too loose and readily slip out of focus. 



The stage should be flat and rigid, either rectangular or circular, 

 so long as it is sufficiently large to accommodate a plate- cultivation. 

 A removable mechanical stage is of great advantage for working 

 with high powers, as a motile bacterium can be constantly kept in 

 view while one hand is engaged in working the fine adjustment 

 (Fig. 17). It may also be employed as a finder if it is engraved 

 with a longitudinal and vertical scale, and provided with a stop. 

 The mechanical stage must be removable, so that the stage proper 



