ZOOLOGY AND BOTANY, MICEOSOOPY, ETC. 551 



heads. The inner face of the slot is so made as to give the razor 

 that inclination which has in practice been found most advantageous. 

 The razor is thus clamped between a flat surface and a screw acting 

 in the middle of the blade, and the edge of the razor is consequently 

 in no way injured. 



The imbedded object is cemented with paraffin into a brass tube 

 which fits tightly on to the end of a cast-iron lever. This tube can 

 be made to slide backwards or forwards, so as to bring the imbedded 

 object near to the razor ready for adjusting. The cast-iron lever is 

 pivoted at about 3 in. from the end of the tube. To the other end of 

 this lever is attached a cord by which the motion is given, and the 

 object to be cut brought across the edge of the razor. The bearings 

 of the pivot are V-shaped grooves, which themselves form part of 

 another pivoted system. 



Immediately under the first pair of V's is another pair of inverted 

 Y's, which rest on a rod fixed to two uprights cast on the base-plate. 

 A horizontal arm projects at right angles to the plane of the two sets 

 of V's ; the whole being parts of the same casting. On the end of 

 the horizontal arm is a boss with a hole in it, through which a screw 

 passes freely. The bottom of the boss is turned out spherically, and 

 into it fits a spherical nut working on the screw. The nut is pre- 

 vented 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 6^ 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 1/25 of 1/6^ 

 or 1/156 in. 



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 

 forwards 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 req^uired 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 on the wheel. By adjusting the position of 

 this sector the feed can be varied from nothing to about 6/32 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, depend- 



