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c.
The Scale of Detection Is Determined by the Scale 
of Manufacture
What we see depends on how we look. From space, the shorelines of the
continents on Earth appear as smooth outlines. Standing on the coastline,
we can see that they are actually composed of jagged racks. The edges of the
rocks may again appear fairly smooth to the naked eye, but examined more
closely with a hand magnifier, we begin to detect roughness.
In examining forensic evidence and deciding which traits are significant,
we must be aware of how the scale we choose for detection may influence
our final conclusion. Take, for instance, a bullet. With the naked eye, the
manufacturing characteristics of diameter, shape, and the number of lands
and grooves are apparent. Depending on the condition of the bullet we may
be able to tell something about other characteristics such as jacketing and
nose construction. Were we to stop at this level of detection, we might
conclude that two bullets that were indistinguishable in all of these charac-
teristics could have been fired from the same barrel. If we then examine both
bullets under a comparison microscope, microstriae become visible. From
these characteristics, we might decide that the two bullets were still indistin-
guishable, even at the magnification offered by the comparison microscope.
We might now decide to examine the bullets under a comparison scanning
 
Figure 6.3
 
Random patterns in nature.
Nature exhibits an infinite variety of
forms. These unique patterns appear anywhere one cares to look. Can you guess
where in nature these forms were produced? (Clockwise from upper right: zebra
from Ngorogoro Crater, Tanzania; rock formation from Death Valley, CA; Moreno
glacier in Patagonia; fingerprint from an unknown person; sand dunes in Death
Valley, CA; Bristlecone pine, White Mountains, CA).
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Principles and Practice of Criminalistics
electron microscope (SEM).* Lo and behold, we find that if we compare the
edges of analogous striae, the two bullets that were previously indistinguish-
able in the comparison microscope now look radically different. Does this
mean we have negated our previous conclusion of common origin? No,
because we do not expect this level of reproduction of detail from one firing
to the next. The scale of detection is a choice made by the examiner, and
knowledge about the nature of the evidence and the scale of significant traits
is critical in deciding which scale to employ.
We can confound ourselves from at least two different directions. Either
we could use too low a magnification, and fail to detect individual differences,
or we could employ too high a magnification and detect differences that
constitute noise in the system. The correct scale of detection is determined
by the scale of production of the unique trait, the origin of the evidence, to
some extent the history of the evidence, and the question. In fingerprint
comparison, one might use a stereomicroscope to enlarge the pattern to a
comfortable viewing size, but it would not be helpful to look under power
high enough to start resolving the grain of the paper or the texture of the
ink. However, when the question is whether the fingerprint is counterfeit
rather than if the pattern matches the reference, the microcharacteristics of
the paper and the ink suddenly assume greater importance, and a different
scale of detection is required.