Internal Structure, Colour and Luster of Oyster pearl of Japan (Abstract)

Abstract

The following work was undertaken by the writer to ascertain th ecause of the colours of pearl., While several investigators have studied this problems, their studies have largely been based on interference or diffraction of liget on sarface of pearl., As a result of the present investigation, quantitative data on interference of light especially by the inner laminae are presented., Whereas the nacreous matter appears homegeneous to the naked eyes, it consists of numerous layers, which may be detected on a plane polished nearly tangential to the surface, as shown in Fig., 2., It was so polished as the thickness or width of laminae appears broader than on the section through a center, as shown in Fig., 1., The intervals between these layers are measured on this figure, by means of a comparator as 1 in table 1, and the true thickness of laminae are calculated as r(diff., ) in the same table., The mean value of thickness thus obtained is 0., 00036mm., When a section of pearl is observed under the polarised microscope, it is obvious that a refractive index of pearl directed parallel to the polariser is nearly same as that of Canada balsam, while that perpendicular to the polariser is higher than that of balsam., The refractive indices of the pearl determined by measureing the minimum deviation angle of the prism made of the pearl, are : α=1., 5300, β=1., 6814, γ=1., 6854, γ-α=0., 1554., From the optical and physical properties of the pearl, associated with the optical orientation a//nβb//nγ and c//nα, which correspond to those of aragonite, it might be said that pearl consists of numerous concentric laminae, composed of radial aggregates or extremely minute aragonite crystals, of which c-axis, is perpendicular to the surface of laminae., When the pearl is ground on the surface with fine-grained carborundum, its colour nearly disappears, while brilliant iridescence alone hardly remains., The colour thus seems to be due to the interference of light by the parallel lam nae near the surface., In Fig., 4 the parallel layers of aragon to separated by extremely thin films of conchlolin are shown., Inc, dental beams of light are reflected at the sarface of the successive layers, each beam being retarded behind its predecessor by 2dn_m cosθ=λ/2n, where d is thickness of layer, n_m is reflactive index, θ is the angle or refraction, λ is wave-length of light, and n is an even on an odd number., It is obvious by this equation that the interference colours depend on the shickness and the angle of refraction., An instance of calculated results is given in the Table 2., When the light passes through the parallel laminae the interference might be produced by the reflection between the planes of neighboering laminae as shown in Fig., 6., In this case the equation for the interference is 2dn cosθ=λ/2n where θ is the angle of reflection and the calculated results are as shown in Table 3., The pearl seems to owe its colour mainly to the two causes above mentioned., The luster is caused by the reflection of light on the surface and varies according to the nature of the surface.,