Ion-thinned longitudinal sections of a third molar were observed by bright- and dark-field electron microscopy. In bright field, the hydroxyapatite crystallites in enamel appeared as long rods, but, when observed in dark field, the crystallites were rectangular, with a mean length of 321 A.

No abstract

Several investigations [1–4] have been conducted in order to show the applicability of the scanning electron microscope (SEM) to the forensic sciences including the examination of firing pin impressions. These studies have shown, for numerous types of physical evidence, the feasibility and potential of SEM. However, as of now, an in-depth examination of one type of physical evidence has not been done. Thus, it is the purpose of this investigation to do an extensive study of firing pin impressions to determine the reproducibility and reliability of SEM using a number of samples and to develop a criterion for comparing firing pin impressions.

Thin foils of high purity aluminium and an AI-AI203 SAP type of alloy were oxidised in a specially designed hot stage specimen chamber in an electron microscope. Below 450~ amorphous aluminium oxide formed on the foil surface and was first detectable at foil edges, holes, and pits. Islands of aluminium then nucleated in this amorphous oxide. The aluminium islands displayed either a lateral growth with eventual coalescence with other islands, or a reoxidation process which caused the islands to disappear. The aluminium island formation was determined to be related to the presence of the electron beam. A mechanism based upon electron charging due to the electron beam was proposed to explain the nucleation, growth, coalescence, disappearance, and geometry of the aluminium islands.

Recently the scanning electron microscope (SEM) was successfully used for an indepth study of firing pin impressions of semiautomatic pistols [1]. The SEM's great depth of field was shown to reveal detail in the impression (far superior to optical microscopy) which could be successfully used for comparison purposes. It was thus the intent of this investigation to extend the work on pistols to shotguns and rifles, in order to see if their firing pin impressions possessed similar class characteristics and identifying features with repeated firings. In addition to this, the effect of preexisting primer marks on the resulting firing pin impression was also studied.