62 MATERIALS AND PROCESSES OF SLIDE-MAKING 



Three types of solid blades are available. First are those which are "square 

 ground"; that is, in which the main portion of the knife is a straight wedge. 

 Second are those which are "hollow ground"; that is, in which both sides of 

 the knife have been ground away to a concave surface, resulting in a relatively 

 long region of thin metal toward the edge. Third are knives which are "half 

 ground"; that is, knives in which one side is square or flat ground and the other 

 side hollow ground. This last type of knife, which the author prefers, is a 

 compromise. There is no doubt that a square-ground knife is sturdier than a 

 hollow-ground knife, a point of some importance when cutting large areas of 

 relatively hard tissues, but there is also no doubt that a hollow-ground knife 

 can be brought more readily to a fine edge. Where a half-ground knife is em- 

 ployed, the flat side should always be toward the block. Microtome knives 

 must be sharpened frequently, but it is necessary, before discussing how to do 

 this, to give a clear understanding of the nature of the cutting edge itself 



If a wedge of hardened steel were to be ground continuously to a fine edge, 

 as in Fig. 34A, it would be utterly worthless for cutting. After only a few strokes, 

 the fine feather edge, which would be produced by this type of grinding, 

 would break down into a series of jagged saw teeth. A microtome knife or, 

 for that matter, any other cutting tool must have ground on its cutting edge a 

 facet of a relatively obtuse angle, whether it is a square-ground knife, as in 

 Fig. 34B, or a hollow-ground knife, as in Fig. 34C. The process of applying 

 this cutting facet to the tip is known as "setting." It is an exceedingly diffi- 

 cult operation to conduct but one which must be learned by every user of a 

 microtome knife. The actual "grinding" of the blade itself to the correct 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 is in continuous use. The nature 

 and purpose of this cutting facet is best explained by reference to the mech- 

 anism of cutting shown in Fig. 34D. 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 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 

 clearance 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. 



