A Universal Microtome, 287 



thickness of 2 microns vary by not more than +10 per cent, of 

 their thickness, the relative displacement of the knife and object 

 from any cause must be less than i micron (0-0002 mm.). This 

 is a very small amount, and it is clear that the instrument must ]je 

 rigid, and that there must be no shake in any of the parts. Each 

 nioving part must in fact move with great precision along its 

 correct path. The sections are cut by moving some part of the 

 instrument by hand, and even with the most careful use there is 

 some uncertain pressure of the hand at right angles to the direction 

 of the movement of the handle, and there must be some bending 

 of tlie frame. The frame should therefore be rigid. But it is more 

 important still to design the instrument so that this uncertain 

 force has little or no effect in bending those parts which will alter 

 the thickness of the sections. There is also a component of the 

 cutting force at right angles to the plane of the section acting 

 between the cutting edge of the knife and the object. This force 

 is not constant, especially with hard sections. These considera- 

 tions show the importance of rigidity in the knife, the object 

 holder, the moving parts, and the frame. All these parts bend 

 when force is applied to them, and it is of comparatively little use 

 to make the frame stiff if the knife or the object holder is capable 

 of springing under the forces acting on them. 



In some sliding microtomes the knife is clamped at one end 

 only ; this is particularly objectionable when cutting objects 

 emlDedded in celloidin with nearly the full length of the knife in 

 use, as the sections are cut by a part of the knife at a considerable 

 distance from the clamped end, and the knife bends more easily. 

 Clamping the knife at both ends very greatly increases its stiffness. 

 In the new microtome the knife is clamped rigidly at both ends in 

 the adjustable knife holder, which can be firmly clamped to the 

 base of the instrument. 



Orientating object holders are often not sufficiently rigid, and 

 have a large number of joints with possibility of slackness, and 

 when provided with means for tightening such joints have the 

 disadvantage that they must be loosened and tightened after each 

 adjustment. The orientating object holder we have designed for 

 this microtome is new and is very rigid. 



There are other causes of want of uniformity in the sections. 

 In microtomes with large sliding surfaces the irregular distribution 

 and the varying thickness of the film of oil between the surfaces 

 may influence the thickness of the sections. The collection of 

 dust on the oiled surfaces wil] have the same effect. The oxidation 

 of the oil will also cause trouble. 



In the new instrument no oil is essential on any surface con- 

 cerned with the precise guiding of the object, although it is 

 desirable with the view of preventing rust that such surfaces be 

 oiled or greased. The moving surfaces in contact are of small area 



