Crime Library: Criminal Minds and Methods

The Atlanta Child Murders

Under a Microscope

A compound microscope uses light reflected from the surface of a fiber and magnified through a series of lenses, while the comparison microscope (two compound microscopes joined by an optical bridge) is used for more precise identification. A different device, the phase-contrast microscope, reveals some of the structure of a fiber, while the various electron microscopes either pass beams through samples to provide a highly magnified image, or reflect electrons off the sample's surface. A scanning electron microscope converts the emitted electrons into a photographic image for display. This affords high resolution and depth of focus.

Another useful instrument is the spectrometer, which separates light into component wavelengths. In 1859, two German scientists discovered that the spectrum of every organic element has uniqueness to its constituent parts. By passing light through something to produce a spectrum, the analyst can read the resulting lines, called "absorption lines." That is, the specific wavelengths that are selectively absorbed into the substance are characteristic of its component molecules. Then a spectrophotometer measures the light intensities, which yields a way to identify different types of substances.

A combination of these instruments for the most effective forensic analysis is the micro-spectrophotometer. The microscope locates minute traces or shows how light interacts with the material under analysis. Linking this to a computerized spectrophotometer increases the accuracy. The scientist can get both a magnified visual and an infrared pattern at the same time, which increases the number of identifying characteristics of any given material.

The first step in fiber analysis is to compare color and diameter. If there is agreement, then the analysis can go into another phase. Dyes can also be further analyzed with chromatography, which uses solvents to separate the dye's chemical constituents. Under a microscope, the analyst looks for lengthwise striations or pits on a fiber's surface, or unusual shapes — as with the one short and two long arms of the trilobal fibers in the Williams case.

In short, the fiber analyst compares shape, dye content, size, chemical composition, and microscopic appearances, yet all of this is still about "class evidence." Even if fibers from two separate places can be matched via comparison, that does not mean they derive from the same source, and there is no fiber database that provides a probability of origin.

 

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