496 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS occur in the fringe system. An axially existing optic path difference, there- fore, leads to a lateral fringe displacement or fringe shift. The amount of this lateral displacement is directly proportional to the optic path dif- ference introduced by the specimen and can be compared directly to the one-wavelength units of optic path difference given by the distances from fringe to fringe in the undisturbed part of the field of view (13-15). Suit- able instrument controls permit variation of the distance between the interference fringes. This does not, of course, change the existing path difference between them, i.e., one wavelength. The akiustment rather cor- responds to a scale expansion, and the sensitivity of detection is directly proportional to the fringe width. In the most sensitivc setting only one fringe covers the whole field of view. This condition is called homogeneous field or interference contrast. Optic path differences introduced by a speci- men then appear as changes of image intensity, which again are a quantita- tive measure for the phase shift introduced by the specimen. Measurements are taken with compensators, which are calibrated di- rectly in m• optic path and which permit an exact measurement of the lateral fringe displacement, or a photomicrograph is taken and the density recorded in a microdensitometer. At this point it may be well to ask what kind of information the cosmetic chemist can obtain from micro-interferometric methods. Such informa- tion falls into two categories. First, there are the directly measurable data: Measurements of optic path. Measurements of refractive index. Measurements of dispersion. Measurements of geometric thickness. Simultaneous measurement of thickness and refractive index. Determination of total dry mass of an object. Determination of total wet mass of an object. Measurement of mass per unit area in the specimen. Concentration of substances in the specimen. Volume of specimen structures. Second, there are the indirect data which follow from the interpretation of the directly measured values and for which only a few examples can be given here. Measurements of geometric thickness may be used to de- termine the thickness and uniformity of sprayed-on films, swelling effects and behavior of materials, or to measure the depths of scratches caused by polishing agents. A practical example is profile measurements, here ap- plied to a determination of skin smoothness (16, 17). This can be carried out as a nondestructive test by taking a replica of the surface using trans- parent replication materials. Figures 9 and 10 show photomicrographs of skin surfaces, one untreated

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