1. The physical mechanism responsible for the high reflectivity of the argentea of the eye of Pecten maximus has been investigated by light microscopy, electron microscopy, interference microscopy and spectral analysis of the reflected light.

2. The argentea consists of 30-40 layers of high refractive index material (guanine crystals) separated by layers of low refractive index material (cytoplasm). Both high and low refractive index layers have optical thicknesses of approximately one-quarter of the wavelength of the light that the argentea reflects best (blue-green, λ = 530 mµ).

3. The reflecting properties of such a system have been investigated theoretically, and the results compared with the measured reflectivity of the argentea at different wavelengths.

4. The refractive index of the guanine crystals is 1·80.

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