178 THE ESSEX NATURALIST.
deal with it fully would obscure the purpose of this paper; yet to
illuminate what has been said on heterochrosis we must have
some knowledge of the nature and causes of different colours.
These, as they are seen in the different parts of birds, are pro-
duced by pigment, by structure, or by a combination of both.
Thus three classes of colour are recognised.
1. Chemical or absorption colours, which are always due to
colouring matter. Such colours do not vary or change under any
position of the light or eye. Black, red and brown always belong
to this group, orange and yellow mostly, but rarely green, and
never blue. White it is said is never due to pigment. It is
produced in the absence of pigment by the texture of the feather,
which forms a fine network, which detracts and reflects light.
The gloss of feathers independent of the colour itself is the
result of their having smooth and polished surfaces.
2. Objective structural colours, which result from the
combination of a particular pigment with a special structure of
the superimposed colourless parts. The use of the word "colour-
less" here presents difficulties, for unless these parts were of a
highly developed transparency they would hardly be colourless
This class of colour includes violet and blue, green almost always,
and occasionally yellow. Such a feather, when examined under
transmitted light, that is, held against the light, appears only in
the colour of its pigment. If, for example, we thus examine the
deep blue or green feathers of a parrot, they will then appear
only grey or yellowish. To demonstrate further the nature of
these objective structural colours it may be stated that blue has
not been observed as a pigment.
3. Subjective structural, prismatic or metallic colours. It is
difficult to give a practical and simple description of this type of
colour, but they are said to change according to the position of the
light in relation to the eye of the observer, and they always change
in the order of the colours of the rainbow. All metallic feathers
are said to appear black when their surface is parallel to the rays
of the light. These colours are produced to some extent by the
structure of the radii, which are composed of one row of compart-
ments which often overlap each other like curved tiles. In the
inside, black or blackish-brown pigment is collected. Each
compartment is covered with a transparent colourless layer of
extreme thinness, which seems to act like a number of prisms.