736 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Figure 7 Chromaticity chart showing positions of skin samples and dominant wavelengths.

MEASUREMENT OF SKIN COLOUR IN VIVO 737 Increased skin pigmentation causes little change in dominant wavelength but the presence of erythema produces a marked shift towards the red region of the spectrum (from 583-590 nm-Fig. 7). Purity (saturation) also shows a marked difference between European and Asian skin. Experiments on small areas of erythema induced on the inner surface of the forearm by contact with detergent solutions indicate that changes in reflection relative to control sites are shown mainly by the red primary. Thus the time course of the development of erythema, and recovery, can be readily expressed by plotting red values against time. As with all visual instruments the Tintometer is no more sensitive than the human eye (unlike some photoelectric devices), but registers quantita- tively visually discernible differences in colour and brightness. VISUAL STANDARDS Many colour atlases have been prepared for a wide variety of purposes. The main disadvantages possessed by colour atlases are the enormous number of standard specimens required to match all noticeably different colours, and the fact that there is often a difference in texture between the standards and the surface being matched. Another disadvantage is that the colours of the standards may suffer from fading or discoloration. In the limited area of skin colours, which cover only a small range of hues, tl•e idea of a colour atlas becomes attractive, especially for field work because of its simplicity in operation and portability. The best known colour atlas is the Munsell system, in which colours are represented in a three dimensional solid. A central vertical scale represents neutral colours with black at the bottom and white at the top. Distance from the central axis represents the saturation of a colour. Differences of hue are represented by the different planes around the central axis, the whole solid being divided into 100 hues. The Munsell system is presented as a book, called the Munsell Book of Colour, with pages representing in- dividual hues. Each page carries a set of chips for the particular hue arranged in a series of columns covering a range of saturations and each column consists of a series of chips of a particular saturation covering a range of lightnesses. A full description of the Munsell and other similar systems, is given by Judd and Wyszecki (1t3). A selection of pages from the Munsell book is published for judging skin colours, but an examination of the book by this author has shown that the colour chips are not close enough together to match many of the large