430 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS If one takes the 2 measurements from a hair and places them into the solu- tion vector formula, a predicted value of the force in grams at 10 per cent de- formation can be obtained. With this model, the coefficient of determination is 0.76. Thus, more than 76 per cent of the variance of the dependent variable, force in grams, can be explained. This value is significant as is ex- pressed by the high overall F-ratio (228, • 2, 140), and low P-value ( 0.001). With the future insertion of additional pertinent variables, as well as a greater sample size base, an additional amount of the unexplained random error can be explained. For more detail about this type of model see (14). From Fig. 13, it can be seen that both Young's Modulus and nmnerical bi- refringence are similarly correlated negatively with relative humidity. A change in relative humidity, or more precisely, a change in water content, inversely changes both the mechanical elasticity and optical anisotropic properties of the hair. Cosmetic conditioning, or treatments which affect the condition of hair, change the hydrogen and salt bonding arrangements of the matrix, and even the more thermodynamic bonds of the entire cortex, in the case of harsher avafiable cosmetic treatments (7). As a result, there is a change in the mechanical stress-strain characteristic, which is the traditional parameter of measurement, as well as a change in the refractive index of the matrix (1, 3, 7, 9, 16, 20). This causes a change in the form birefringence, in addition to a possible change in the fibrils themselves, causing a change in the intrinsic birefringence (3, 4, 11, 19). Again, numerical birefringence is an extremely sensitive measure of molecular orientation in an anisotropic material (3, 5, 21). Changes in mechanical elasticity or condition are a result of changes in molecular bonding and orientation (1, 7, 9, 16, 20). In that light, stress-strain analysis appears to 'be measuring a secondary parameter. The mechanical method of testing is inherently obtrusive. Required are both an optical device for size determination and a tensile device for mechani- cal determination. Any inconsistency or flaw over the span of material ana- lyzed tends to bias the results (1). To build a system of analysis, it is necessary to use certain known parame- ters of evaluation as a tangible reference to which a correlation can be es- tablished. In the case of numerical birefringence, mechanical stress-strain analysis has served this function. Numerical birefringence is presented as an alternative quantitative system of analysi.s of human hair condition at the molecular level. With this tech- nique, one is able to determine, unobstrusively, utilizing only one instrument, at specific areas on the shaft, molecular occurrences associated with hair con- dition and conditioning.

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