452 



XXXII. CELL-DIVISION AND NUCLEAR DIVISION. 



means of clamps, the blade of an ordinary hollow-ground razor, 

 while that rather more to the right serves as a fulcrum for a bent 

 lever (horizontal in the figure) which carries the long oscillat- 

 ing arm occupying the top of the figure. This bent lever has its 

 long arm moved up and down by means of the vertical screw seen 

 near the right-hand end of the figure, and when the long arm of 

 the lever is moved up the oscillating arm resting upon it is thrown 

 bodily forwards, and vice versa. By means of the handle at the 

 extreme right an adjustable paul is actuated which engages the 

 teeth of- the milled head of the vertical screw, raising the long arm 

 of the bent lever through a minute fraction of an inch, depending 

 in amount upon the number of teeth the paul had been adjusted 

 to miss, and therefore throwing forward the oscillating (or rocking) 



FIG. 159. Cald well's Rocking Microtome. 



arm through a fraction of this fraction of an inch. The rocking 

 arm carries the object embedded in its paraffin block, fused on to 

 a permanent paraffin block contained in a socket in the rocking 

 arm itself; and the same movement which, in its last stage, 

 actuates the paul also lowers the rocking arm by means of a cord 

 and carries its paraffin end upwards past the razor, while in its 

 recovery, brought about by a strong spring, the object to be cut is 

 thrust against the razor edge, and the section is cut. The value 

 of one tooth upon the milled head which the paul engages is 

 stated at TT7 ^ of an inch (-000625 mm. or -625 /*) ; and if this 

 theoretical value is maintained, sections of any required thinness 

 can be cut in sequence. The number of teeth the paul should 

 carry forwards at each movement is readily adjustable. 



