Pattern evidence (or here) is the oldest branch of forensic science, although it's use as individual evidence. Individual evidence is any object which can be said to come from a one particular source. There is no chance (no whatsoever, wink) that it came from any other source. The pattern evidence covered in Jay Siegel's beginner's guide are:
Other kinds of individual evidence include paint chips, fractured fragments of glass and of course DNA. Individual evidence should be compared to class evidence. Class evidence can not be associated with any one particular person or source. Class evidence is still useful of course. Hair and fibers are class evidence. So is ink and dye or certain physical characteristics of drugs. Show prints or tire tracks might be class evidence. Class evidence is tied to groups not to individuals.
Finger prints, forensic ballistics (a term the author has a problem with) and handwriting are all supposed to be individual evidence. They are all supposed to be able to identify whether or not a potential source has produced the piece of evidence.
Despite the fact that forensics has been using certain types of pattern evidence for over 100 years, it can justifiably be criticized. Pattern evidence can grouped into the so called “experience-based forensic sciences”. Since there can be no absolutely quantitative analysis of these, forensic “scientists” who work with these types of evidence rely on experience and judgment. That's hardly what I would refer to as hard science.
Finger prints are pretty cool. You have them on your fingers, palms, toes and souls. We've all stopped to look at them in our life but have you ever wondered what they might be for? They add traction to your grip so that you can hold on to things better. The ridges appear during the eighth week of a pregnancy and remain unchanged (other than growing in size, unless of course they are artificially altered which by the way just makes identification that much easier, duh) throughout a persons entire life. The root of the ridges is in the middle layer (called the dermal papillae) between your dermis and epidermis.
Finger prints are not determined by your genes (cough... not completely determined by your genes). Identical twins have easily distinguishable prints. People argue that it is impossible for two people to have the exact same set of finger prints (although how they would have legitimate statistics on this is beyond me). In any case, finger prints are usually an acceptable form of individual evidence.
When the surface of your skin comes in contact with a surface (glass, plastic but even paper and fabric) biological (like perspiration, skin cells or proteins) and alien (like ink or oil) material is left behind forming a latent print. If the print is visible to the naked eye and needs no amplification, it is called a patent print, such as if it were left in paint or blood. If the print is actually impressed into some matrix (like wax) then it's called a plastic print.
When they are latent (i,e, invisible) they must be found. Smooth surfaces can be dusted or fumed with superglue. Magnetic powders can be used for surfaces with fine texture. Dusting isn't the only way. Chemical fuming with iodine creates temporary reddish images which must be photographed. Silver nitrate can be applied to latent prints as a aqueous solution but therefore can't be used on surfaces that can't get wet. Ninhydrin can be sprayed onto latent prints. It reacts with amino acids to forms a a colored compound. Ninhydrin even works on porous surfaces (i.e. paper!). Laser luminescence can also be used.
So they can find them but what do forensic scientists see? Fingerprints are made up of various topographical elements.
1) The print will have a sort of pattern. There are four major patterns: either the ridges loop, arch, tent or whorl. The four general types are typically broken down into eight patterns which can be seen in the figure.
2) Besides the general pattern there are little details in the ridges, called “minutiae”. Ridges can do all kinds of things:
Bifurcations - when ridges split (trifurcations occur too)
Ending Ridge – self explanatory
Dot – a tiny round island
Short Ridge – a small isolated segment
Enclosure – when a ridge bifurcates and then reconstitutes
3) And looking at even more detail (under a microscope) gland pores, the shape of edges or little scars can help to compare prints
As a final note finger prints can be used to identify some sorts of drug use by individuals.
This was a little more extended than I would have like. Tomorrow I'll cover handwriting and ballistics.