30 SUMMABY OF CURRENT RESEARCHES RELATING TO 



finally by both causes combined; (2) Subjective colours are caused 

 by the various effects of broken or reflected light. 



The colours owing to the presence of pigment are always black, 

 brown, and red of various shades ; only one instance is known of a 

 green colour produced by pigment, and that is in the feathers of the 

 Touracous. The violet and blue tints are never due to pigment 

 alone, and often depend merely upon lines and grooves on the surface 

 of the feather. There are numerous colours which appear to be due 

 to the combination of definite pigmentary bodies within the substance 

 of the feather, and the structure of the feather itself, and this is the 

 case especially with blue feathers. If one of the blue feathers of a 

 Macaw be pressed and broken so as to destroy its structure it appears 

 to be of a brownish grey colour, which is owing to the presence of pig- 

 ment of that colour. Dr. H. Gadow has published some interesting 

 observations upon these colours. He finds that the blue feathers of 

 many birds consist of an outer structureless sheath, beneath which is a 

 layer of " cones " covered by a system of extremely fine lines running 

 parallel with the long axis of the cone ; below these cones lies a layer 

 of brownish-yellow pigment, which appears black when present in 

 great quantity. The whole surface coating of the feather varies 

 not only in different birds, but in the different feathers of the same 

 bird, and is in any case too thick to allow of the blue colour being 

 explained in the way that other colours are produced by thin plates. 

 The fine ridges upon the cones seem to be the source of the blue 

 colour. 



The colours of yellow feathers are sometimes due simply to the 

 presence of yellow pigment ; but since many yellow feathers contain 

 no pigment, this explanation will not hold in every case. In all pro- 

 bability a system of fine lines observed upon the outer surface of the 

 feather is the cause. Similar lines occur in violet feathers, but they 

 are finer and not quite so straight, and in this way, perhaps, the 

 difference in colour is produced. 



With regard to the green colour of many feathers, the suggestion 

 of Krukenberg, that it is caused by an admixture of yellow pigment 

 and a blue optical structural colour, is not a sufficient explanation in- 

 asmuch as most green feathers do not show the same peculiar 

 structures that are met with in blue feathers. All the green feathers 

 examined show the foUowiug structure : a transparent smooth sheath 

 covers the barbs and barbules ; beneath this is a system of ridges and 

 fine pits ; the ridges are less regular than those of the yellow coloured 

 feathers ; beneath this layer is yellowish or brownish pigment. 



The second group of colours (subjective) are produced by a 

 transparent sheath which acts as a prism. They are the so-called 

 " metallic " colours, which change according to the position from which 

 they are viewed. In describing the colours of birds a good deal of 

 confusion has arisen from this fact, and Dr. Gadow suggests the 

 desirability of introducing a standard method of describing these 

 metallic colours in order to insure uniformity, and gives a diagram 

 illustrating three _ positions in which the bird should be placed in 

 order to describe its colours. 



