86 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS •6 Glear Lacquer ,•00 ,• •0 5•0 5•0 6oo 65o 7oo Wovelen•7th (nrn) Figure 5. Reflectance curves of guanine plates from herring scales and swimbladders thickness is approximately 130 nm. The curve for the swimbladder crystal reveals much lower reflectance it is typical of very thin films which reflect bluish white. The swimbladder platelets have an estimated opti- cal thickness of 36 nm based on the shape of the reflectance curve. The geometrical thickness is thus 36/1.85 or 19 nm, in the range found by electron microscopy. The difference between the two types of crystals is inherent in the apparent functions of these crystals in the fish. Denton has shown that the reflection of the guanine crystals in the scales and skin of fish provides camouflage for the fish (7). The platelets are oriented in such a fashion that in daylight the fish seen from any position (except directly below) reflects the amount of light which matches that of its surroundings, and is thus effectively hidden. The thickness of the pearl essence platelet is such as to produce maximum reflectance. Reflection is not pertinent to an internal organ like the swimbladder, a major function of whi.ch is to expand and contract with gas to vary the buoyancy of the fish. In the case of the conger eel, Denton et al. report that it is the silvery guanine- containing layer which makes the swimbladder relatively impermeable to nitrogen, oxygen, and carbon dioxide (8). Permeability is presumably

WHITE NACREOUS PIGMENTS 87 reduced because the oriented platelike particles greatly increase the length of the diffusion path. Crystals which fulfill this function need not be reflective. Indeed, a given amount of guanine is used more efficiently if the crystals are thin. These examples show how reflectance is dependent on platelet thick- ness, in agreement with interference behavior. They also illustrate the amazing ingenuity of biological processes which modify crystal dimen- sions, as required, to perform diverse functions. SUMMARY Reflectance curves for thin films, thinner than those which produce interference color, have been calculated from a form of the Fresnel equa- tions dealing with interference behavior of thin films. Spectrophoto- metric curves at specular reflection were then measured for a series of ti- tanium dioxide-coated mica pigments with different TiO2 layer thick- nesses. The experimental curves were found to conform very closely in shape to the theoretical curves for interference films. The similarity demonstrates that the behavior of even "white" nacreous pigments is pri- marily an interference phenomenon. ACKNOWLEDGMENT The authors wish to express their gratitude to Mr. Louis Armanini for the preparation of the special TiO2-coated mica samples, and to Mr. Philip Greenberg for the preparation of the sample of swimbladder crystals. (Received June 7, 1971) RWFERWSCWS (1) Grecnstein, L. M., Nacreous pigments and their properties, Proc. Sci. Sect. Toilet Goods Ass., 45, 20-6 (May, 1966). (2) Vaslcek, A., Optics o[ Thin Films, North-Holland Publishing Go., Amsterdam, 1960, pp. 99, 122-32. (3) Greenstein, L. M., and Bolomey, R. A., An instrumental study of the optical characteris- tics of nacreous pigments and interference pigments, J. Soc. Cosmet. Chem., 22, 161-77 (1971). (4) Linton, H. R. (to E. I. du Pont de Nemours and Go.), Nacreous pigment compositions, U.S. Patent 3,087,828 (1963). (5) Quinn, C. A., Rieger, C. J., and Bolomey, R. A. (to The Mearl Corp.), Method of coating surfaces with high index oxides, U.S. Patent 3,437,515 (1969). (6) Miller, H. A., and Bienes, H., private communication (19õ1). (7) Denton, E. J., On the organization of reflecting surfaces in some marine animals, Phil. Trans. Roy. Soc. London, Set. B, 258, 285-313 (May, 1970). (8) Denton, E. J., Liddicoat, J. D., and Taylor, D. W., hnpermeable "silvery" laycrs in fish, J. Physiol. (London), 207, 64-5P (January, 1970).