Much may be learned on this point from careful inspection of the edge, and trial on a piece of horn, such as an old razor handle. When drawn over a clean piece of horn once or twice the edge of a soft knife is completely dulled; if well-tempered it should scarcely lose its keenness. Again, when laid flat on the thumb nail and pressed, the edge ought to bend up without breaking or crumbling, and at once, when the pressure is removed, resume its original shape. The extent to which extreme hardness and durability, or toughness, may be combined, is well shown in the famous Toledo sword-blades. One of these blades will shear through an iron nail without having its edge perceptibly dulled, and yet so tough and springy is it that it may be coiled up into a hoop of several folds without breaking or receiving any permanent set. If we could only get section knives of stuff like this, it would be a pleasure to work with them.
After all, however, the only test of these two points, material and temper, is a fair trial in actual practice. Of the form of the blade, however, it is easy to judge, and there are a few points which are frequently overlooked and which give rise to errors that are attributed to other causes.
In the text we are told that “It is essentially necessary that the back and edge of the blade be strictly parallel to each other.” The author undoubtedly knew what the correct form should be, but the definition he has given is not a correct mathematical statement of the conditions involved. These are as follows:
The edge must form a straight line, and both the edge and the under side of the back must lie in the same plane.
Let us consider these conditions somewhat carefully.
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.
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.