Refractive Index Determinations of Glass Fragments. A Simplified Procedure

Abstract

Particles of broken glass constitute one of the more commonly found types of physical evidence. Their occurrence results primarily from the fact that glass so frequently serves as the only physical barrier to the criminal bent on burglary, that bottles are frequently used as weapons of assault, and that the automobile is necessarily equipped with glass encased lights in a region most likely to suffer damage on impact. The breakable nature of the material, its tendency to shatter when broken, the tenacity with which particles of it may be held by the usual fabrics, and the apparent ease with which the fragments find their way into pockets or trouser cuffs, all serve to assure its continuing value as physical evidence. The real value of glass as evidence, however, lies in its physical characteristicsnotably density and refractive index-and the fact that both of these characteristics vary over relatively broad ranges, depending upon the composition of the material. Considering the great variety of glasses which are produced, and the manufacturing tolerances or unavoidable differences in composition which result from production methods, we are not surprised to find that a given glass has a considerable degree of individuality as judged by density and refractive index determinations. Fortunately for the criminalist, both density and refractive index can be determined with a rather high degree of accuracy. More important, however, are the still more accurate comparative techniques, wherein a particle of evidence glass may be directly compared with fragments from a suspected source. Methods utilized here (particularly those concerned with density determinations) are capable of distinguishing extremely small variations between samples. As to refractive index determinations, it cannot be said that the comparative techniques provide a degree of accuracy (as contrasted with absolute values) comparable to that obtained in density determinations, but any comparison of glass fragments must necessarily involve a determination of both density and refractive index before a conclusion of identity may be formed. Although considerable work has been done in recent years in the development of improved procedures and techniques as regards density determinations, little or no improvement appears to have been made in respect to refractive index determinations. It appears that many criminalists do not routinely attempt to determine the absolute refractive index value of glass fragments, and those who do are not too satisfied with their results, even realizing the limitations of any absolute-value method.