5.15.
EXAMINATION AND SELECTION OF GLASS KNIVES
161
almost invariably can be seen arching toward the scored side, and commonly intersects
the edge near that corner. The region of intersection should be avoided when sectioning. A, B, and C are illuminated differently with reflected light. All show to the right
that the fracture deflected as it approached the edge, resulting in a convexly curved
surface involving about half of the knife edge on the side opposite the score. More or
less severe checkmarks also commonly are to be found in this zone. The relatively
obtuse edge angle of this region limits good sectioning, and this portion of the knife
should be used only for facing the block. The best part of the knife for ultrathin
sectioning is just to the right of the intersection of the fracture ridge and the edge.
Here the angle will be most acute, and the edge free of defects.
edges. However, one wishes this degree of acuity at all times so that the
gross diagonal angle should not exceed 45°.
5.15. Examination and Selection of Glass Knives
Reflected light is useful for examining knife edges. When one has
confidence in his technique, probably nothing more than a cursory
examination at low power under a binocular dissecting microscope will
be used to see if gross defects spoil a potential edge. By twisting and
turning the knife in a light beam, check marks intersecting with the edge
can be seen without much difficulty. For a more critical analysis of an
edge the light can be manipulated so that it yields a "dark-field" effect.
The edge is then seen as a bright line against a dark background [Fig.
17(c)]. The knife edge, or part of it, which is very acute will then appear
as an extremely thin illuminated line (Fig. 17, portion labeled "most
acute"). Wherever the intersecting fracture was badly deflected, the convexly curved surface so formed reflects as a relatively broad band (Fig. 17,
"deflection"). This type of illumination also will show up check marks
that commonly are associated with (and fortunately limited to) the rolled
portion of the edge. Thus, one can see exactly what parts of the edge can
be expected to be best for sectioning. Sheldon (1957) describes how to use
a compound microscope with an objective lens equipped to provide
annular incident light to examine glass knives. This makes possible a
particularly good evaluation of the edge.
Naturally, one likes to have as long a length of good knife edge to work
with as possible, so there is point in trying to improve techniques which
further this aim. However, it is foolish to be overly concerned with perfection. Many people seem to become almost paranoid about this in
relation to knife making. The fact of the matter is that almost any
EXAMINATION AND SELECTION OF GLASS KNIVES
161
almost invariably can be seen arching toward the scored side, and commonly intersects
the edge near that corner. The region of intersection should be avoided when sectioning. A, B, and C are illuminated differently with reflected light. All show to the right
that the fracture deflected as it approached the edge, resulting in a convexly curved
surface involving about half of the knife edge on the side opposite the score. More or
less severe checkmarks also commonly are to be found in this zone. The relatively
obtuse edge angle of this region limits good sectioning, and this portion of the knife
should be used only for facing the block. The best part of the knife for ultrathin
sectioning is just to the right of the intersection of the fracture ridge and the edge.
Here the angle will be most acute, and the edge free of defects.
edges. However, one wishes this degree of acuity at all times so that the
gross diagonal angle should not exceed 45°.
5.15. Examination and Selection of Glass Knives
Reflected light is useful for examining knife edges. When one has
confidence in his technique, probably nothing more than a cursory
examination at low power under a binocular dissecting microscope will
be used to see if gross defects spoil a potential edge. By twisting and
turning the knife in a light beam, check marks intersecting with the edge
can be seen without much difficulty. For a more critical analysis of an
edge the light can be manipulated so that it yields a "dark-field" effect.
The edge is then seen as a bright line against a dark background [Fig.
17(c)]. The knife edge, or part of it, which is very acute will then appear
as an extremely thin illuminated line (Fig. 17, portion labeled "most
acute"). Wherever the intersecting fracture was badly deflected, the convexly curved surface so formed reflects as a relatively broad band (Fig. 17,
"deflection"). This type of illumination also will show up check marks
that commonly are associated with (and fortunately limited to) the rolled
portion of the edge. Thus, one can see exactly what parts of the edge can
be expected to be best for sectioning. Sheldon (1957) describes how to use
a compound microscope with an objective lens equipped to provide
annular incident light to examine glass knives. This makes possible a
particularly good evaluation of the edge.
Naturally, one likes to have as long a length of good knife edge to work
with as possible, so there is point in trying to improve techniques which
further this aim. However, it is foolish to be overly concerned with perfection. Many people seem to become almost paranoid about this in
relation to knife making. The fact of the matter is that almost any