142 BIRD GALLERY. 
Colour.—The colour of feathers is due to one of three causes :— 
(1) It may arise from the presence of actual pigment, (2) from pigment 
overlaid by colourless structures, or (3) from iridescence due to the effect 
of light falling on the polished, ridged or pitted surfaces of the feather 
which act as prisms. 
Fig. 4. 


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Side view of Right Foot of a Purple Gallinule (Porphyrio) to show the 
composition of the horny covering (podotheca). 
1. Hallux or hind toe. 4. Outer toe. 
2. Inner toe. 5. Seales (Scutelle). 
3. Middle toe. | 6. Reticulate scales. 
Yellow colour, like black, brown and red, is as a rule due to pigment 
diffused throughout thesubstance of the feather. Some yellow feathers, 
however, contain no pigment whatever, the colour being due to the 
