A Method for Evaluating Glass Knives

In microtomy, faults of the cutting edge are an obvious and avoidable source of artifact. The purpose of this communication is to describe a simple apparatus for viewing and marking the cutting edge of the glass knives which are used in see-tioning for electron microscopy (1, 2). The apparatus consists of a monocular microscope with base, arm, body tube, coarse and fine adjustments, an annular incident light objective lens, and a mechanical stage that has been modified to hold a glass knife. The apparatus is illustrated in Text-fig. 1. The advantage of the annular incident light illumination is that rough surfaces reflect light into the objective and appear bright; an effect opposite to that obtained when strictly vertical illumination is used. A facet without faults appears as a narrow, white line in a dark field, while a faulty or dirty edge is a speckled, widened, or fissured white line. Three different knives are shown in Figs. 1 to 3. We have found a 50 × objective lens combined with a 15 × eye-piece satisfactory for discriminating between cutting edges. The glass knife is mounted vertically between a rigid pillar and thumb screw allowing adjustment of the angle of viewing. This vertical mounting together with the length of the objective lens make it necessary to increase the usual working distance between the objective lens and the mechanical stage. To that end a block was placed in the arm of the microscope above the mechanical stage. However, a microscope with a vertically adjustable stage and adequate range of motion as well as the annular incident light illumination and objective lens is commercially available? The glass knife holder is mounted on a plate which is screwed to the usual slide-holding mechanism of the mechanical stage. This provides for systematic lateral and transverse motion of the knife. While moving the knife laterally to examine the edge it is necessary to adjust the focus continually because of the slight curvature of the cutting edge. When a region of the cutting edge is found to be free of faults, its exact location may be marked by introducing the diamond point until it touches the glass below the knife edge. The diamond marking pencil is mounted just below the level of the glass knife edge and its mounting is fixed to the stage itself in line with the midpoint of the objective lens. Screws permit precise adjustment …