292 Transactions of the Society. 



fit is geometric are : — The bottom bearing of the micrometer screw ; 

 the connexion of the micrometer screw nut to the end of the feed 

 lever ; the fulcrum of this lever ; the fulcrum of the swinging 

 arm; the connexion between this lever and the swinging arm 

 carrying the object; and the clamp for the sphere carrying the 

 object holder. 



Vertical Movement of the Object. 



As previously mentioned a difference of 10 per cent, in the thick- 

 ness of two successive sections amounts to * 0002 mm. with sec- 

 tions • 002 mm. thick. It is clear that the nut on the micrometer 

 screw must transmit its movement to the object with great 

 accuracy. Now there must be friction in the vertical movement 

 of the swinging arm on its knife edges. The first very small 

 movement of the micrometer screw will move the nui a very small 

 amount ; the first effect will be to bend the lever connecting the 

 nut to the sliding part, and this bending will go on till there is 

 sufficient force to overcome the friction of the slide. It would 

 seem that the relation between the movement of the object and 

 the micrometer screw might not be perfect. The danger of this 

 error is increased because the nut is connected to the slide by a 

 lever, and this must have more spring in it than when the con- 

 nexion is more direct. We considered this point and came to the 

 conclusion that it would not cause an appreciable error for the 

 following reason, and experiments proved we were right : — 



The vertical sliding part is also the axis of rotation of the 

 swinging arm. Now it is well known that a rotating shaft even 

 when carrying a heavy wheel can be moved endways by the 

 slightest force ; although when it is at rest the force ret|uired to 

 move it is great. We all know that less force is required to pull 

 a cork out of a bottle if the cork is rotated at the same time, and 

 most of us have done this without realizing the reason. The same 

 thing happens in the vertical axis of the microtome ; the micro- 

 meter screw is turned and the swinging arm rotates ; this causes a 

 slight movement of the rubbing surfaces in the vertical slide in a 

 direction at right angles to the direction in which the sliding parts 

 should move. This eliminates the effect of friction so far as it 

 acts in the vertical direction in which the sliding motion takes 

 place, and the ratio of the movement of the micrometer screw to 

 the vertical movement of the object is constant. 



A defect in many microtomes is that either the diameter of 

 the micrometer feed screw is too small or that the pitch is very 

 fine, and not infrequently both of these defects are present in the 

 same instrument. In the first case the screw is readily bent, and 

 in the second case the amount of wearing surface between the nut 

 and the screw on which it works is small, and the wear soon 



