Scene of the Crime (14 page)

2. The perp's hands must have been fairly moist, and must have well-developed ridges.

3. Minimal time must have passed. Twelve hours is about the absolute maximum if there is to be any chance of success.

The silver plate or the photographic paper is clapped down firmly,
in one motion,
on the victim's skin where s/he remembers having been touched. It is lifted off—again in one firm motion—and carefully dusted. In the case of the photographic paper, the developed latent is then covered with tape. However, this procedure produces the mirror image of a print lifted in the normal manner.

The silver plate is used the same way. When the print is transferred to a lifting card, it will produce a mirror image of a normal print. In order to work with either comfortably, you will need to make a photographic negative and then an internegative and then use the internegative the way you would normally use a fingerprint lift. See Figure 4-5 for an explanation.

In general, photographic paper is just as efficient and is far cheaper and easier to work with. According to the Tokyo Police Department, the soft tissue X ray provides better results. Unfortunately, as I said, it simply is not generally available in the United States. Your supercop can use it if s/he wants to.

Where to Look for Prints

Ident people spend a lot of time watching the way people use their hands. Notice, next time you are writing, the way your palm and little finger curl into a reverse C—or into a C, if you're left-handed— and rest on the paper. Those are the parts of the hand that leave prints on a forged check. Next time you start to drive an unfamiliar car, notice how you automatically adjust the rearview mirror. That's the best place to find prints in a stolen car—not the steering wheel,

on which the prints pile up on top of each other until they're totally illegible.

Figure 4-6 gives the names of the parts of the hands. The
thenar zone
and the
hypothenar zone
turn up most often on documents, especially on the backs of checks. The
palmar zone,
which is divided into several numbered areas for ease in discussion, turns up when something has been grasped. The thumb and the first three fingers turn up on opposite sides of something that has been grasped with the fingers rather than with the whole hand. The little finger almost never is found in crime scenes.

The
interpalmar zones
show up in various situations; again, the best way to figure out where to look for what, is the same way ident people do it: Watch the way people use their hands. If you are trying to decide for fiction what part of the hand would leave prints, actually put your hands through the actions your fictitious perp would have used. (This is done very well on "Murder, She Wrote." I've enjoyed watching Angela Lansbury twisting her hands around, regarding them with wide-mouthed and wide-eyed calculation.)

Hew Do You Know What You've Found?

I told you I knew that print on what turned out to be Leon McCoy's revolver was a thumb print as soon as I saw it. How did I know? How do you work with those prints from crime scenes?

We need another chapter for that.

TABLE 4
_

The Latent Fingerprint

These pages come from
The Finger Print Manual,
published by Sirchie FingerPrint Laboratories, Inc., and are used by permission. The charts have been edited for use in this publication.

Latent fingerprints generally consist of a mixture of natural secretions from glands in the skin. Often these latent prints contain contaminants that are picked up on the skin (some fingerprints consist solely of contaminants).

Determination of the major constituents of a fingerprint by simple visual examination is usually impossible, unless the fingerprint is in an obvious contaminant (blood, grease, dust, etc.).

Distribution of the major constituents of secretions of the sweat glands and the actual distribution within a given latent fingerprint vary from donor to donor, and can greatly vary with the same donor at any given time (large variations occurring from day to day and even minute to minute).

Sweat Gland Secretion

There are three major sweat glands found in the human body: the eccrine glands, the sebaceous glands and the apocrine glands. The secretions of these glands are as follows:

Most natural fingerprints consist of a mixture of sebaceous and ec-crine sweat. Some of these chemicals persist for long periods of time in latent fingerprints while others may decompose, evaporate or diffuse.

The presence of a specific chemical in a fingerprint depends on the constituents of the original fingerprint, the nature of the surface, the time elapsed since deposition, and the storage conditions. Factors such as temperature, exposure to light and water and the relative humidity affect the chemical and physical nature of a fingerprint.

Water is the first component to be lost from most fingerprints. Because of this, when dealing with prints more than a few days old, processes which primarily detect water are less effective than those processes which detect primarily the fatty component.

Detection of Latent Fingerprint Constituents

Some fingerprint detection processes are specific to particular chemicals, while others detect the oily or greasy physical nature of the surface.

Since the chemical and physical nature of a fingerprint is generally not known before the fingerprint examination, the techniques used and the order in which they are applied are determined primarily by the particular surface type the fingerprints are on.

Methods for Latent Fingerprint Development

Powders:
Many powders for the development of latent fingerprints are available. With fresh fingerprints, the aqueous component of the fingerprint contributes significantly to the adhesion of powders, while with older prints, powders adhere principally to the fatty deposits of sebaceous sweat. In many cases, flake powders such as Silver Latent Print Powder are more sensitive than other types of powder. In some instances, a less sensitive powder may be more effective.