“The parts of the Feathers of this glorious Bird appear, through the Microscope, no less gaudy then do the whole Feathers; for, as to the naked eye ’tis evident that the stem or quill of each Feather in the tail sends out multitudes of Lateral branches, … so each of those threads in the Microscope appears a large long body, consisting of a multitude of bright reflecting parts.
… their upper sides seem to me to consist of a multitude of thin plated bodies, which are exceeding thin, and lie very close together, and thereby, like mother of Pearl shells, do not onely reflect a very brisk light, but tinge that light in a most curious manner; and by means of various positions, in respect of the light, they reflect back now one colour, and then another, and those most vividly. Now, that these colours are onely fantastical ones, that is, such as arise immediately from the refractions of the light, I found by this, that water wetting these colour’d parts, destroy’d their colours, which seem’d to proceed from the alteration of the reflection and refraction.”
“The finely colour’d Feathers of some Birds, and particularly those of Peacocks Tails, do, in the very same part of the Feather, appear of several Colours in several Positions of the Eye, after the very same manner that thin Plates were found to do in the 7th and 19th Observations, and therefore their Colours arise from the thinness of the transparent parts of the Feathers; that is, from the slenderness of the very fine Hairs, or Capillamenta, which grow out of the sides of the grosser lateral Branches or Fibres of those Feathers.”
“The colours of animals are due either solely to the presence of definite pigments in the skin, or … beneath the skin; or they are partly caused by optical effects due to the scattering, diffraction or unequal refraction of the light rays. Colours of the latter kind are often spoken of as structural colours; they are caused by the structure of the coloured surfaces. The metallic lustre of the feathers of many birds, such as the humming birds, is due to the presence of excessively fine striae upon the surface of the feathers.
“In many biological systems such as insects, fish, birds and flowers, the common mechanism for the most brightly coloured appearances is coherent scattering from photonic crystal structures. Study of these systems is informing a broad range of sciences, providing not only functional, behavioural and materials understanding, but also suggesting protocols with which light, colour and appearance manipulation in technology may be more effectively embraced. This presentation will offer an overview of the field of biological photonics, and will present in detail several recent discoveries that reflect nature’s optical design ingenuity, combined with some technological applications for which they are currently being developed.”
Abstract from “Evolutionary light manipulation in biological systems” seminar, presented at University of Sheffield, December 2015, Peter Vukusic
I recently attended a seminar that was led by Prof Vukusic from the University of Exeter, part of the Soft Matter and Biology Physics Group series of seminars – the abstract can be read above.
What I learned during this fascinating presentation – and ensuing discussion – is that our perception of colour in many insects, fish and birds is determined by exceedingly tiny structures that interfere with waves of light as they strike the surface, altering the angle and length of these waves. This affects the distance that these wavelengths travel, and the colour that we are likely to perceive, before they reach our eyes. We sometimes talk in poetic terms about the play of light upon a sunlit surface – and in this case these structures act almost like a finger that bends a plucked string to play a note of a particular pitch. Irridescence could be considered as the shimmering play of a series of adjoining notes – an arpeggio of colour.
These crystalline structures are of almost staggering beauty and complexity and remain mysterious in many ways – at the moment we can only guess at the cellular mechanisms by which they are constructed. Their purpose is often a matter for speculation – although many are clearly linked to expressions of biological fitness. Biological nano-sculpture is nothing if not sexy.
What is fascinating from an aesthetic point of view is that these structures depend on greater or lesser degrees of order to achieve their brilliant effects. Certain colonies of bacteria will produce structural colour effects, for example, that depend on the organisation of the individual units within the colony – the more organised the distribution of individuals, the more definite the structural colour effect.
Within more complex organisms we see disordered, spongy nano-structures scattering light to create distinctly white colour effects (white being the ultimate admixture of all colours). Recent work by Andrew and his team has also highlighted the very organised arrangements that create the consistent impression of blue that we observe on the wing of the Eurasian jay (Garrulus glandarius).