ADULT AND CHILDREN'S HAIR 167 with each year of age for the ages of 6 to 18. No significant sex differences were found. Curiously, the basis for the change in color is hardly discussed in the literature except as the comment, "adults appear to generate more pigment" not a very satisfactory explanation. In view of the apparent continued change with time over the full life span of the individual, a specific hormonal basis is unlikely. Physical and Morphological Effects on Color The appearance of hair depends, of course, on the optical physic of thc situation--the nature of the incident light and the way it i reflected, absorbed, and scattered by structural clements of the fiber and by the geometrical arrangements of the hairs as a group. On the latter point, it is a fact of common experience that a neatly parallel array of fibers is conducive to high luster and brightness if the angles of the incident light and of the eye are properly disposed a tousled head of hair is generally dull in appearance. Some facts on the physics of hair color, following the views of Garn (29), may be instructive for general background. Light that falls on hair is absorbed to a large extent (70-95%), a small amount is transmitted through the fiber, and from 2-20% is reflected--the last being most important in the hair's appearance. The reflected light has two components: (a) that reflected from the surface of the fiber, and (b) that re-reflected after absorption. The surface-reflected light is plane- Table VIII Hair Color as a Function of Age • Mean Hair No. of Age Color Cases 6 5.5 80 7 6.4 178 8 7.6 224 9 8.7 218 10 9.2 246 11 10.2 267 12 11.1 285 13 11.9 304 14 12.5 307 15 13.5 302 16 14.3 219 17 15.2 128 18 15.7 43 "After Steggerda (28).

168 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS polarized and contributes to the sense of sheen or gloss smooth coatings like oils increase the mirror-like reflectance, and particulate deposits such as dirt, detergent residue, or hair spray particles decrease it. A smooth cuticle and a regular cross section lead to maximum reflectance. Greater scaliness and the presence of lateral ridges along the shaft (said to be present in some Negro hair) or a crenulated, irregular cross-sectional shape result in a more diffuse reflection pattern. The reflected light has the same spectral char- acteristics as the incident light--"the hair shines with the same color as the illuminant"--and thus the tonal characteristic of the innate hair color is diluted. The absorbed light that is re-reflected is selective with respect to wave- length. The maximum absorption is in the near UV, about 400 m/•, and the minimum absorption is at 700 m• and beyond. In the visible region, the curve of reflectance with wavelength is quite linear over the range of 400- 700 m/•, with a slight upward curvature for reddish tone hair (30). Other things being equal, two pigment characteristics influence the reflectivity after absorption: the size of the pigment particles and their density of distribution. These factors plus the depth of the pigment bed--i.e., the length of the absorption path--affect the color. Darkening with aging may be related to the larger diameter discussed earlier, but there is also evidence for increase in size and number of pigment granules as well. A small fraction of light transmitted through the hair exhibits a different spectrum from that for absorption and reflectance it is redder due to scattering by the pigment granules. Since the incidence of medullation increases with aging, scattering from this cause would be expected to increase and to result in reddening of the hue with age. It may be desirable here to sketch an over-all basis for the existence of various kinds of natural hair color. Color in hair is produced by pigment particles (brownish-black melanin granules) dispersed in a clear transparent matrix of hair keratin substance. The principal difference in the appearance with respect to blondness or brownness is a consequence of the number of pigment granules present. A high density of pigment leads to the appearance of brown or dark hair and a low density of pigment to blonde. An analogous situation is seen in screen printing, where the density of black dots on a printed page affects the visual appearance of darkness or lightness. It seems reasonable to suppose that production of melanin pigment increases with age, and thus more granules per fiber are present in older people. in addition to the melanotic brown pigment, there may also be present a much more diffusely dispersed or "soluble" red-gold stain throughout the