“You see, but you do not observe”
The Adventures of Sherlock Homes, A Scandal in Bohemia
By Mike Barrett
Forensic science is nothing more than the application of scientific technology in support of the legal system. Forensic science typically includes disciplines such as DNA typing, toxicology (drugs & poisons), questioned documents, trace evidence (hair & fiber), blood alcohol, crime scene investigation and firearms identification.
Contrary to the images portrayed in popular television shows such as CSI™, the real forensic scientists are impartial judges of the evidence placed before them. Kicking in doors and arresting the “bad guys” is not generally part of their job description. They are advocates for the evidence.
The focus in forensic science is to obtain answers to two fundamental questions:
- “What is it?” and
- “Where did it come from?”
The mechanism to answer these two questions varies with the specific item in question; however, all the forensic disciplines have in common a set of well established practices, procedures and equipment to facilitate their task of identifying and linking.
This forensic specialty seeks, among other things, to determine a common origin for fired bullets and expended cartridge cases. The discipline came to the forefront of forensic science as a result of the laboratory examination of fired bullets and expended cartridge cases arising from the infamous “Saint Valentine’s Day Massacre” in 1927.
The principal governing the discipline is that of tool mark generation. It states that when two surfaces are brought together under pressure, the harder surface will mark the softer surface. The nature of the marks produced will be a function of:
- The relative hardness of the two surfaces
- The magnitude of the pressure applied
- The relative movement of the two surfaces
- The nature of the two surfaces at their points of contact
The markings produced by a given tool on a substrate are unique and reproducible. Numerous studies have been undertaken over the decades, which have confirmed this principal.