158 THE STORY OF ANIMAL LIFE 



as regards the quality' of the light they reflect. 

 The secret of the difference lies in the greater 

 opacity of the feathers named ; they are black 

 feathers, while those of the parrot are light- 

 coloured. Now after the metals themselves, 

 there are few objects in nature so opaque as the 

 black pigment of a black feather. If a thin 

 section through the roots of young black feathers 

 is cut for examination under the microscope, 

 the pigmented parts, although cut so very thin, 

 appear completely opaque. And just as a glass 

 gives a better reflection when backed by some- 

 thing opaque, so does the reflecting surface of 

 the feather. Hence it is that the quality of the 

 colours reflected by these feathers is what we call 

 "metallic." If we ask for a definition of this 

 metallic brightness, other than the accepted fact 

 that it resembles the light reflected from metals, 

 the artist will reply that it consists in two things 

 (i) the greater brilliancy of the light reflected, 

 that is to say the greater completeness of the 

 reflection ; and (2) the entire absence of those 

 gradations of light which are afforded by the 

 reflections from any object, however dark, that 

 possesses a surface translucent, even in the 

 smallest degree. " Metallic " reflections, in fact, 

 may be defined as those in which the greatest 

 amount of light is reflected, and the reflected 

 sunlight receives from the reflecting surface the 

 least possible degree of modification. While the 

 actual tint of the colour reflected by a black 

 feather, then, is determined by the form and 

 position of its angular ridges, the quality of 

 the reflection is determined by the opacity of 

 the substance itself. It is interesting to note 

 that the opacity necessary for reflecting a " me- 



