ORIGIN AND CHARACTERISTICS 21 



second place we have colour arising from a pig- 

 ment in combination with the overlying colourless 

 structures in the form of irregular longitudinal 

 ridges or polygonal bodies, between the surface 

 of the barbs and barbules and the pigment. 

 When the polished surface of such feathers is 

 scratched or held up against the light only the 

 colour of the pigment is visible. This is the case 

 with many blue, yellow, orange, and green 

 feathers. In the third place we have those 

 wonderful prismatic or metallic colours, of which 

 the radiant Humming-Birds offer the most 

 striking example. In these feathers a black or 

 blackish-brown pigment rests below a transparent 

 colourless layer, the surface of which may be 

 either smooth and polished, covered with longi- 

 tudinal ridges or sprinkled with innumerable pits, 

 which serve as a series of prisms or facets, and 

 change the colour of the feathers according to 

 the direction from which the light falls upon 

 them. It is interesting to know that these 

 prismatic colours change their hue in the same 

 order as those in the rainbow. When the light 

 glides in a vertical direction over the surface of 

 all these metallic feathers, the luminous rays are 

 absorbed and they appear black. But, as Martin 

 observes, respecting the plumage of Humming- 



