Thin-layer interference and diffraction mechanisms for the Tyndall phenomenon, quantified by Rayleigh's law and possibly by Raman's molecular diffusion are good candidates for our search of iris color's determinants. 06 Pigment-management structures evolve independently in the iris, which is an exceptional structure, because it is not covered with skin, hair or feathers like all the others. They account for homeothermic vs. poikilothermic differential selection pressures. The iris is also the only organ bathing simultaneously in the external and the internal environments. Last, but not least, the iris has defied researchers when it was discovered, back in 1996, that its melanogenesis, thought to end in early childhood, is a lifetime long, continuous process.
Irides do change their color.
Archetypically the simplest model for melanogenesis, the blue iris, is the only one where color has a straightforward explanation. Irides colored hazel enough to turn blue-green when humans migrate in seaside environments as they seasonally do in France, remind us of mimicry mechanisms otherwise obvious in chameleons. The r elative part of the two mechanisms determining iris color, structural and pigmentary, rests undetermined. Iris studies should address all the details of iris cells' functional anatomy. Melanosomal physiological, pharmacological and behavioral characteristics, including origin, aggregation, number, content, control, disappearance in apoptosis, pathological cell physiology, are all areas of immediate interest.