Making Sections 143 



whether it is a square-ground knife or a hollow-ground knife (both 

 are shown in Fig. 107). The process of applying this cutting facet 

 to the tip is known as "setting." It is an exceedingly difficult opera- 

 tion to conduct but one that must be learned by every user of a mi- 

 crotome knife. The actual "grinding" of the blade itself to the cor- 

 rect angle or to the correct degree of hollowness cannot be done in a 

 laboratory; the knife must be returned to the manufacturer or to some 

 scientific supply house equipped with the special machinery necessary. 

 The cutting facet, however, must be set at least once a day if the blade 



4 



CUTTING TIP 



••CLEARANCE 

 ANGLE 



KNIFE 

 ANGLE 



BLOCK 



Fig. 108. Diagram to show cutting action of knife on wax block. 



is in continuous use. The nature and purpose of this cutting facet are best 

 explained by reference to the technique of cutting shown in Fig. 108. 

 Notice first that the knife blade itself must be inclined at such an angle 

 to the block that the cutting facet is not quite parallel to the face of the 

 block. There must be left a "clearance angle" to prevent the knife from 

 scraping the surface every time that it removes a section. In cutting wax, 

 this clearance angle should be as small as possible, and it is for this reason 

 that the blade holder of a microtome is furnished with a device for setting 

 the knife angle. The knife angle should not be set with reference to any 

 theoretical consideration but with regard only to securing this small clear- 

 ance angle. The only way to judge whether or not a satisfactory clearance 

 angle has been obtained is to observe the sections as they come from the 

 knife. If the clearance angle is too large, so that the section is not being 

 cut from the block but is being scraped from it, the section will have a 

 wrinkled appearance and will usually roll up into a small cylinder. If the 

 clearance angle is too small, so that the lower angle of the facet is scrap- 

 ing the block after the tip is passed, the whole ribbon of sections will be 

 picked up on the top of the block, which will itself crack off when the 

 knife point reaches it. It is obvious that the knife angle will be changed 



