302 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS done in this area, and the results suggest that scale patterns are sufficiently characteristic to allow broad classes or species to be distinguished, i.e. a human hair can be readily distinguished from a cat hair, but not a dog hair from a cat hair. An Atlas (16) of hair cross-sections, longitudinal views and scale patterns is available as a reference work. However, scale counts (number of scales in unit length) do not provide a means of distinguishing human hairs as the variation across a single human head is similar to that across the population, a similar distribution to that in Fig. 1 is obtained. Amino-acid analysis of hair The amino-acid composition of human hair does not show any signi- ficant variation from person to person (Bogaty (17)) and is therefore unlikely to provide a means of hair characterization. The amino-acid analyses of wool, human hair, animal hairs and finger-nails are all known to be similar. Pyrolysis gas chromatography This has been investigated by two workers (De Forest (18) and Fou- weather (19)) but does not provide an effective means of hair discrimination. The pyro grams of all the individuals studied were essentially similar and any small differences between individuals were within the experimental error. A comparison of two pyro grams from two individuals is shown (Fig. 2). Physical parameters of hair A study (Dabbs (20) and Greenwell, Wittmer and Kirk (21)) has been made of the refractive index (RI) of human head hairs and, whilst the RI varies from person to person, the variation of RI across a single head reduces the discrimination to 1 in 2/3, e.g. in a typical case studied an individual dis- played hairs of RI from 1.546 to 1.550 compared with the variation over the population from 1.542 to 1.555. However, in view of the limited data avail- able for hair characterization, it does at least provide a method of dividing the hair population in two.

HUMAN HEAD HAIR AS FORENSIC EVIDENCE 303