It is difficult to imagine a real forensic science laboratory without a microscope. In the examination and analysis of trace evidence, the polarizing microscope is a necessary essential and no full-service forensic laboratory in the world would be without at least one and a scientist who can use it. The investigation of fibers, sand and soil, paint, glass (yes, even though it is isotropic), explosives and complete unknowns (e.g. “white powders”), to list a few of the more common types of microscopic evidence, depend on this instrument and its ability to recognize and identify virtually any transparent or translucent particles that might be encountered. Man-made fibers exhibit uniaxial (and sometimes biaxial) optical properties that lead to their rapid identification and comparison. By its means, a single calcite or silicon carbide crystal can be identified with certainty and its morphology characterized. X-ray diffraction could not even detect such crystals when present in a dust sample among the numerous other particles present. The list of microscopic traces goes on indefinitely, with each new case bringing some new type of particle or residue to be studied, characterized, identified and compared.

It is regrettable that it is increasingly difficult to find academic training in optical crystallography (let along a well-trained optical crystallographer) and as a result, forensic scientists are increasingly either self-trained or receive instruction in short courses offered by various organizations – other than academic institutions. The author received some of his training in the subject from Prof. Bloss and many of his forensic students have had the benefit of learning their petrographic microscopy using the “Bloss approach” to the subject.

This presentation will attempt to illustrate, by means of real-life casework from the presenter’s 40 years of experience in forensic microscopy, how the polarizing microscope and its use by a well-trained and experienced analytical microscopist has become indispensable in the analysis of microscopic particles when they occur as evidence in serious crimes.