698 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS standard procedure for preparing and cleaning the skin should be used which preserves the skin in as near normal conditions as possible.* Operator/subject interaction The operator carrying out the measurements should try to maintain a tranquil atmosphere for example by keeping verbal contact to a minimum. Where possible the subject should be fully briefed before measurements are carried out. Where the operator presses a probe against the skin the pressure should be kept steady by using a constant force connection between probe tip and the instrument. This will permit an acceptable range of relative move- ment between the operator and subject. Effect of site-to-site variation It is common practice in product evaluation to use adjacent skin areas for control and product comparative tests. However, there may be an intrinsic variability between the two sites and there are preferred areas of the body where the variability is acceptably small. In particular the volar area of the forearm shows high homogeneity for a wide range of physical properties such as cohesion (18), squame cell diameter (19) and sweat rate (20). Due regard should be paid to the existence of isotropic contractive stress lines--Langer's lines (9). These can manifest themselves in preferred directional changes in topography when the turgidity of the skin is changed. The accommodation period With protracted cosmetic skin treatments there exists the possibility of a change in the skin to give a new state of quasi-equilibrium so long as the treatment persists. After the treatment ends a change in the property measured may be observed. This effect may be ascribed to adaptation through chemical insult (21). Accordingly, for critical evaluation, suitable controls should be em- ployed so that an additive's effect may be properly distinguished. * The following solution was found suitable for cleaning the skin. 4 ml of 46 % I.M.S., 50 •o water, 2 •o Ernpicol TL40, 2 • P.E.G. 300 was applied to a 2 in. square lint cloth and the skin was wiped with this. This was followed by gently wiping the skin with a fresh dry lint and then waiting 10 rain before measuring.

OBJECTIVE EVALUATION OF COSMETIC PROPERTIES OF SKIN 699 APPLICATION OF EVALUATION PRINCIPLES TO THE MEASUREMENT OF SKIN SMOOTHNESS The nature and perception of smoothness From the previous discussion several principles become self-evident whenever in vivo instrumental evaluation is required to be meaningful in consumer terms. It will be shown later in this paper how the application of these principles can help in obtaining an understanding, in scientific terms, of a most im- portant cosmetic attribute of skin, namely 'smoothness'. The perception of skin smoothness and the physical factors associated with smoothness are complex, as indicated in studies of 'feel'. The wide range of interest in perception of feel was shown by the A.S.T.M. study of the feel of materials and hand-operated lever systems. Some of the earliest discussions of feel include Zigler's paper on 'Stickiness' (22), where he emphasized the importance of both viscous friction as well as attractive forces at solid interfaces. The cosmetic aspects of the feel of films of pro- ducts on the skin have been discussed by many other authors (23-28). The feel of things has been examined in a broad sense by Harper (29) and he and Stevens have published a detailed psycho-physical treatment of bulk feel including the contribution from hardness (30). Compared with anisotropic systems, the feel or surfaces of layered and textured materials has received relatively little attention. For example Stellwagen, Hasing and Rubert have considered texture, but no physical analysis in terms of frequency, amplitude and direction was given (31-33). The contribution of friction has been considered by several workers such as Naylor (34) but usually only in simple terms using classical laws of friction which cannot wholly apply to skin--see, however, Appledoorn and Barnett (35). The neuro-physiological processes of touch have received a great deal of attention and Jenkins gives an excellent review where he describes im- portant aspects like adaptation, discrimination, complex properties like greasiness and kinesthesis (36). The transmission of skin sensations has been discussed by Wall (37) and yon B•k•sy (38). Yon B•k•sy (38) stresses the importance of the second differential of the stimulus in relation to the ultimate perception magnitude. Edge or contrast effects are in general felt more intensely than a steady stimulus magnitude. Thus, in feel, the stickslip