142 The Preparation of Microscope Slides 



complex a microtome is used; if the knife-edge is not perfect, there is no 

 chance of obtaining a perfect section. Ordinary razors are not satisfactory 

 for the production of fine sections. It is necessary to buy a microtome 

 knife, preferably from the manufacturer of the microtome. Another type 

 of microtome knife employs the edge of a safety-razor blade in a special 

 holder; this does not, in the author's opinion, give as good results as a 

 solid blade. 



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

 ground," that is, those in which the main portion of the knife is a straight 

 wedge. Second are those that are "hollow ground," that is, those 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 that are "half ground," that is, knives in which one side is 

 square- or flat-ground and the other side is 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. 106, 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, 



106 



CUTTING 

 FACET 



BLADE 



107 



Figs. 106 and 107. Types of cutting edge and cutting action. Fig. 106. Simple wedge 

 without cutting facet. Fig. 107. Flat- and hollow-ground blades showing cutting facet 



