Making Sections 145 



This direction for the use of three strokes in setting applies, of course, 

 only to a knife that has been treated reasonably and not to one which 

 through carelessness has acquired a nick in its edge. Where the nick is 

 large, it is almost impossible to remove it in setting because the continual 

 setting merely grinds away the edge of the knife and ultimately alters the 

 thickness of the blade itself. If the knife-edge is nicked to a deeper 

 extent than about % mm, the only thing to do is either to return the 

 knife to the manufacturer to be reground or to avoid that portion of the 



Fig. 110. Stropping a microtome knife. The knife is drawn smoothly backward once 

 or twice on each side. 



blade containing the nick when cutting sections. It must be emphasized 

 that the only purpose of setting is to produce a cutting facet, and that 

 grinding, which cannot be done in the ordinary laboratory, is required 

 for the removal of knife imperfections. The next question is that of "strop- 

 ping" the blade of the knife by pulling it backward across a leather surface 

 in the manner shown in Fig. 110. If the knife has been set properly, 

 stropping, the only purpose of which is to polish the facet, is quite un- 

 necessary. The nature of the leather surface that is used for stropping 

 makes it obviously impossible to pull the knife blade forward, and there 

 is a grave risk in pulling it backward that the facet, instead of becoming 

 polished on its flat surfaces, will become rounded on its edges and thus 

 undo the work of setting. Certainly no beginner should be permitted 

 to use a strop until he has demonstrated his ability to set a knife-edge 

 to the point where it will cut an excellent section without stropping. It is 

 also strongly recommended to the beginner that he examine the edge of a 

 knife under the low power of a microscope before setting, after setting, 

 and after stropping. 



