There are many kinds of colour in animals and these are due to the reflection of some, although not all, of the components of incident white light.  A coloured object reflects some of the components and the actual colour depends upon the wavelengths on which they reach the retina of the observer's eye.

Some colour is structural, that is to say that the physical nature of its surface is responsible for the colour.  Other colour may be pigmentary, that is to say it is caused by the presence of pigment, and some colour may be due to a combination of both structure and pigment.  Structural colours are caused by interference, diffraction or scattering of light and examples can be given of the colours of a soap bubble or the colours in mother of pearl, or the blue of the sky. In fact, soap bubble colour is caused by the reflection of light from both outer and inner surfaces with that from the inner surface travelling further.  If the distance between the inner and outer surfaces is such that optical retardation is an odd-number of half wavelengths then part of the spectrum is cancelled out and the observer sees the sum of the remainder.  As the distance between the surface varies in different parts, the result is a play of colours and these vary with the angle from which the bubble is observed.  This colour change is due to interfering rays of light.  In fact, colours vary with the angle of vision in a very definite order known as Newton's series and such changes of colour occurring with differences in eye position constitute iridescence.

Iridescent colours can also be produced by a diffraction grating.  These can be seen if a series of parallel equidistant scratches are made very close together on a polished metal surface.  On either side of a beam of white light falling across the scratched lines there appear spectra giving an iridescent appearance to the surface.  In mother of pearl superimposed layers formed by tablet-shaped crystals of aragonite are cemented together by very thin layers of an organic substance and lie inclined to the inner face of the shell.  As they strike against the shell surface their parallel laminae form a curved diffration grating.

In some mammals, coloured light is reflected by part of the coat of the layer in the eye that contains blood vessels and pigment (situated immediately outside the retina).  Light that would otherwise be absorbed by dark melanin pigment is reflected; the effect can be seen at dusk and is known as eye shine.  It is thought that these reflected colours are due, in some instances at least, to diffraction.

Structural colours produced by interference or diffraction change according to the angle from which the object is seen but there is another kind of structural colour which looks the same from any angle.  This is produced by the scattering of the shorter waves in white light by very tiny particles and is the sort of colour already described in the blue of the sky.  If the particles are small, they scatter more of the short wave than of the long wave components of white light and in the spectrum the violet and blue end is much more scattered than the red end.  (The spectrum ranging from the short end to the long end as follows: violet, blue, green, yellow, orange, red).

There are two sorts of scattered colour and in this discussion colour includes black, white and their intermediates.  One sort of scattering occurs in white hair.  Whiteness of hair is like whiteness of snow or of lillies; it is caused by air spaces in a solid translucent substance.  The air spaces lead to light-scattering but there are no pigment granules or other substances to absorb light.  If it was possible to produce white hair with air spaces very very minute then the colour effect would be blue.

There is a special name for the sort of scattering that produces colour by the presence of very minute particles.  In 1869 John Tyndall studied and explained the phenomenon of the blue colour of the sky and henceforth the light scattering producing blue has been known as Tyndall scattering. Tyndall scattering is the type of scattering that produces the blue sky colours already mentioned, the blue colour of human eyes and the blue colour of some cats' eyes.  In 1866 a researcher stated that blue eyes are blue from a turbid medium in front of a dark background and, later, C. W. Mason found that minute protein particles in the iris which have a higher refractive index than the surrounding stroma (connective tissue) scatter the incident white light and thus cause blueness.  A gradual increase in the size of the particles with advancing age causes a gradual lessening of the Tyndall effect and thus a gradual lightening of colour.  Light is apparently scattered against pigment at the back of the iris which prevents the red colour of the blood in the capilliaries from overriding the more feasible blue.  Albino animals which lack the layer of melanin therefore have pink eyes.  In Tyndall scattering, a dark background increases the intensity of blue; this is seen in the bright blue of the African lizard where melanin pigment behind the protein particles which scatter again increases the intensity of blue.  Another frequently quoted example is that of the bluish chin of dark-haired men seen after shaving.  One explanation is that the colour is due to the black souls of men coming out on their chins every morning!  The other, more scientific explanation is that of Tyndall scattering --without the black roots of the hairs for a background the feeble Tyndall scattering would not be seen.  In fair men, where there is no dark background, the bluish effect is not seen.

(This subject will continue in Study Unit 20.  Readers wishing to write to Miss Turner should address their letters to her at One High Cross, Framfield, E. Sussex, England.  Personal replies cannot be undertaken but questions will periodically be answered by Miss Turner in Cat World Magazine.)