174 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS where G.O. stands for growth of hair out of H. The change in C' within H due to cuticle wear can be expressed as hange in C' per unit time due tøl FAC'I uticle leaving H due to wear = = [17] Flow (•) Figure 10) L At ] w w where W stands for wear. The net change in C' within H per unit time can then be ex- pressed as: or ß .NET \dt /o.,. L3 at/oo. Substituting [15] and [16] in [18] we have: noting that [ 19] becomes [c:] = cid, ,[ca+•,] = ci•+•, ,•: 7:1• a.dx [18] [ 19] t /N.T dx dt \ dt/w or (ac, / =_ ac,.ax (ac, / dt /NET dx dt \ dt /w C' is (to a good approximation) proportional to C therefore, dC' = KdC hence NET -- d--•'dt •- w which is equation [2] in the text of the paper. REFERENCES (1) R. F. Stature, M. L. Garcia and j. j. Fuchs, The optical properties of human hair I. fundamental con- siderations and goniophotometer curves, J. Soc. Cosmet. Chem., 28,571 (1977). (2) R. F. Stature, M. L. Garcia and J. j. Fuchs, The optical properties of human hair II. the luster of hair fibers,J. Soc. Cosmet. Chem., 28,601 (1977). (3) L.J. Wolfram and M. K. O. Lindeman, Some observations on the hair cuticle, J. Soc. Cosmet. Chem., 22,839 (1971).

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