Section-Cutting / A Practical Guide to the Preparation and Mounting of Sections for the Microscope, Special Prominence Being given to the Subject of Animal Sections

by Sylvester Marsh

[19]. Holtzapffel tell us that chisels that are required for paring across the end grain of moderately soft wood are considered to hang better to the work when they have a very slight keen burr or wire edge thrown up on the face or flat side of the tool. But this does not apply to section knives.

When the knife is to be honed, the back is applied and fastened by means of two or three set screws. When laid on the hone, the edge of the knife and the lower surface of the back form the guide and regulate the cutting angle. In this way we can use a broad, thin blade and yet secure great accuracy in honing it. Such a blade has this advantage also, that it is not so liable to be rounded and thus injured by stropping as one in which the relief is obtained by hollowing out the sides.

Fig. 9.

Such, then, are the general principles which govern the construction of cutting tools, including the section knives used by microscopists. We will now give a few practical directions for selecting a knife and putting and keeping it in order.

In selecting a knife for cutting sections, regard must be had to the texture of the material that is to be cut. To attempt to cut delicate sections of soft tissues with the stout knives which are suitable for cutting sections of woody fibre would be to destroy the sections, while to reverse the operation and cut sections of wood with thin, delicate knives would result in the ruin of the knife. We have seen a most excellent knife seriously injured by an attempt to cut material that was too hard for it. The knife was very thin, and had proved most excellent in cutting sections of such material as kidney, liver, etc. An attempt was made, against our protest, to cut a section of an apple-shoot, the wood of which was mature. Before the knife had gone half way across, it bent, dug into the wood and broke, leaving an ugly gap in the middle. Experienced section-cutters know this very well, but young microscopists are not so familiar with the facts just stated, and the point is too important to be overlooked. Those, therefore, who devote themselves to microscopical studies, or who expect to make sections of materials of several kinds, differing in hardness, etc., must provide themselves with knives of different degrees of strength.

For common work, good razors are as good as anything, provided they can be obtained with straight edges. Where razors are not suitable, recourse must be had to the surgical instrument maker, though we are sorry to say that there are but few in this country that know how to forge, temper and grind a decent knife. Most of our dealers in instruments do not make the instruments they sell; they import the goods they sell with their names stamped on, and thus get a reputation as manufacturers; a special order they are unable to fill respectably. There are some exceptions, but of the majority of dealers what we have written above is true.

The points which specially demand attention in a knife for cutting sections are these: 1. Quality of the steel used; 2. Temper; 3. Form of the blade.

Of the quality of the material of which a knife is made, nothing can be determined except by actual trial. The old tests of staining with acids, examining with the microscope, etc., are worthless, or at least too crude and uncertain to be of any practical value. Color changes with the degree of polish that is produced, and, in short, there is no reliable guide. The purchaser must depend entirely upon the reputation of the manufacturer. There is plenty of good steel to be found; the trouble lies with the cutlers. They are careless and in haste, and as a consequence they burn the steel or fail to work it sufficiently, and the result is a useless tool.

The steel may be of the very best quality, however, and well forged, and yet the knife may fail from being badly tempered—too soft or too hard. If too soft, the edge is soon dulled; the knife requires to be frequently honed, and the time wasted in keeping it in order is a serious drawback. If too hard, it is impossible to give it a keen edge, for the metal crumbles away as soon as it is honed or stropped very thin, and the edge becomes ragged and dull. Good steel, well forged, may be so tempered that it will neither crumble nor become rapidly dull.