52 THE MICROSCOPE 



should be of the lever pattern, preferably the Nelson 

 (Fig. 43). In this form the unequal length of the 

 arms of the lever secures very delicate movement, and, 

 moreover, only a small portion of the weight of the 

 body tube is transmitted to the thread of the vertical 

 screw actuating the movement. 



A spindle milled head (Fig. 44) will be found a very 

 useful device to have fitted in place of the ordinary 

 milled head controlling the fine adjustment. In this 

 contrivance the axis of the milled head is prolonged 

 upward in a short column, the diameter of which is 

 one-sixth of that of the head. The spindle 

 can be rapidly rotated between the fingers 

 for medium power adjustments while the 

 larger milled head can be slowly moved 

 when focussing high powers. 



5. The stage (Fig. 40, e) should be square 

 in shape and large in area at least 1 2 cm. 

 flat and rigid, in order to afford a safe 

 support for the Petri dish used for plate 

 Spindle head to cultivations; and should be supplied, with 

 fine adjust- spring c iip S (removable at will) to secure 



the 3 by i glass slides. 



A mechanical stage must be classed as a necessity 

 rather than a luxury so far as the bacteriologist is 

 concerned, as when working with high powers, and 

 especially when examining hanging-drop specimens, 

 it is almost impossible to execute sufficiently delicate 

 movements with the fingers. In selecting a mechan- 

 ical stage, preference should be given to one which 

 forms an integral part of the instrument (Fig. 45) 

 rather than one which needs to be clamped on to an 

 ordinary plain stage every time it is required, and its 

 traversing movements should be controlled by sta- 

 tionary milled heads (Fig. 45, A A'). The shape of the 

 aperture is a not unimportant point; it should be 

 square to allow of free movement over the substage 



