USING THE ROCKING MICROTOME 471 



screw passes freely. The bottom ot the boss is turned out spheri- 

 cally, and into it fits a spherical nut working on the screw. The nut 

 is prevented from turning by a pin passing loosely through a slot 

 in the boss. The bottom of the screw rests on a pin fixed in the 

 base-plate. 



It will be seen that the effect of turning the screw is to raise or 

 lower the end of the horizontal arm, and therefore to move backwards 

 or forwards the upper pair of V s, and with them the lever and 

 object to be cut. The top of the screw is provided with a milled head, 

 which may be used to adjust the object to the cutting distance. The 

 distance between the centres of the two pivoted systems is 1 in. and 

 the distance of the screw from the fixed rod is It} in. The thread 

 of the screw is 25 to the inch ; thus, if the screw is turned once round. 



the object to be cut will be moved forward of . or in. 



25 <ij 15tt 



The turning of the screw is effected automatically as follows : 

 A wheel with a milling on the edge is fixed to the bottom of the 

 screw ; an arm to which a pawl is attached rotates about the pin 

 which supports the screw. This arm is moved backwards and for- 

 wards by hand or by a cord attached to any convenient motor. 

 When the arm is moved forward the pawl engages in the milling 

 and turns the wheel : when the arm is moved back the pawl slips over 

 the milling without turning the wheel. A stop acting against the 

 pawl itself prevents any possibility of the wheel turning, by its own 

 momentum, more than the required amount. The arm is always 

 moved backwards and forwards, between two stops, a definite 

 amount, but the amount the wheel is turned is varied by an adjustable 

 sector, which engages a pin fixed to the pawl and prevents the pawl 

 from engaging the milling of the wheel. By adjusting the position 

 of this sector, the feed can be varied from nothing to about - 3 4> of a 

 turn ; and hence, since the screw has 25 threads to the inch, the 

 thickness of the sections cut can be varied from a minimum, 

 depending on the perfection with which the razor is sharpened, to a 



maximum of '-- of - of - , or - of a turn. The practical mini- 



.) -jD Ojr 1UUU 



mum thickness obtainable with a good razor is approximately .f-,,, 1 ,,,,, 

 inch. The values of the teeth on the milled wheel are as follows :- 



1 tooth of the milled wheel = ^^^ in. = -000625 mm. 



2 teeth '-^^ in. = -001250 mm. 

 4 ., =I -i_in. = -002-5 mm. 



16 ^nr-'no in. = -01 mm. 



The movement of the lever which carries the imbedded object is 

 effected by a string attached to one end of the lever. Tins string 

 passes under a pulley and is fastened to the arm carrying the pawl. 

 Attached to the other end of the lever is a spring pulling downwards. 

 When, the arm is moved forward the teed takes place, the string is 

 pulled, the imbedded object is raised past the raxor. and the spring 

 is stretched. When the arm is allowed to move back, the spring 

 draws the imbedded object across the edge of the razor, and the sec- 

 tion is cut. The string is attached to the lever by a screw which 



