ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



667 



endless screw a brings the latter by means of the unilateral pressure of 

 a spring into close contact with the toothed wheel. By means of this 

 pressure-position and the double gripping of the wheel and axle all 

 backlash of both is avoided. The toothed wheel has 60 teeth and 

 requires a half rotation to move the spiral from the indent of the cam 

 to its apex and to perform the 3 mm. of motion. In one rotation, 

 therefore, one tooth corresponds to a movement of * 1 mm. ; and a 

 complete rotation of the axis a secures a complete rotation of the toothed 



Fig. 154. 



wheel. The drum r of the axis is divided into 100 parts : the rotation 

 of one graduation of the drum-head Corresponds, therefore, to a micro- 

 metric movement of 0-001 mm. The connecting piece between tube 

 and pillar carries the rack-and-pinion coarse adjustment and bears on 

 its hinder side a swallow-tailed piece fitting accurately into a correspond- 

 ing groove of the pillar, and firmly screwed on to the bearer Tc. This 

 bearer by means of the roller g sits on the surface of the spiral and 

 shares both its rising and its failing movement. A spring inserted in a 

 cylinder on the pillar of the stand over the support 1c, presses a pin 



