CHAPTER XI 
COLOUR, SONG, AND ALLIED PHENOMENA 
Nature of Colours 
WHILE others were discussing for what purpose the 
brilliant colours of birds existed and what part they 
played in the life of the species thus adorned, some 
German investigators with characteristic thoroughness 
set to work to discover the nature of the colours them- 
selves. They found that the colours in birds’ feathers 
might be divided into two classes. There were first 
those which appeared the same from any point of 
view ; secondly, those which changed as the bird, or as 
the person who watched it, moved. To these classes 
have been given the names of objective and sub- 
jective colours. The first class had to be subdivided 
into two—viz. those which were due to pigment 
alone, and those which were due partly to the pig- 
ment and partly to the feather’s structure. 
Thus we have—(I.) objective colours, (1) due to 
pigment ; (2) due to pigment, plus structure ; (II.) 
subjective colours. 
