Since the hole in which the material to be cut is imbedded, is always circular, and since the blade is generally pressed forcibly against the table of the microtome, it is obvious that if the edge of the knife be curved it will sink into the hole to a greater or less depth according as it is further from the centre of the hole or nearer to it. To explain this let us refer to the greatly exaggerated drawing given in figure 10 where the edge of the knife, _a_, _b_, is shown in two different positions covering the hole of the microtome. The least degree of reflection will show that as the knife passes towards the centre from _a b_ to _aʹ bʹ_, it will have a tendency to turn on the back as on a hinge, and the middle of the edge will sink deeper into the hole as it approaches the centre of the latter. The section will therefore vary in thickness and prove useless. This difficulty can be obviated only by the use of a straight edge like _e f_, which will always lie in the same plane. [Illustration: Fig. 10.] If the knife were perfectly rigid, and the curved edge and back lay entirely in the same plane, this difficulty would not occur to any great extent. But since a curved edge rarely lies wholly in the plane of the back of the knife, and since the blades of most knives, especially those thin ones used for cutting sections of soft tissues, yield a little to pressure, and since this pressure is never exactly the same, it is impossible to prevent irregularities from occurring. While it is not necessary that the back of the knife and the cutting edge should be parallel to each other, it is absolutely necessary that they be in the same plane, or, as mechanics say,—“out of wind.” If this condition be not observed, the blade will rock on the microtome table as it passes over it, and irregularity in thickness of section will result. A slight knowledge of geometry will enable any one to see that these statements are strictly accurate. As regards other points in the form of the knife, we refer to what has been previously said when discussing the general principles which should govern the construction of cutting tools. We now proceed to give a few practical directions for sharpening knives and razors and keeping them in order. Section knives, during the process of sharpening, are subjected to three distinct processes: 1, Grinding; 2, Honing; 3, Stropping.