METALLIC PLUMAGE. 31 



raid-green, but, on changing the light, that is, 

 altering its angle of incidence, the emerald was 

 changed into velvet-black. Audebert considered 

 this changeableness to be due to the organiza- 

 tion of the feathers, and to the manner in which 

 the luminous rays are reflected on falling upon 

 them ; and of this we think there can be little 

 doubt, for each feather, when minutely inspected, 

 presents us with myriads of little facets, so dis- 

 posed as to present so many angles to the inci- 

 dence of light, which will be diversely reflected, ac- 

 cording to the position of the feather, and in some 

 positions not reflected in any sensible degree, 

 and thus emerald may become a velvet-black. 



Lesson supposes that the brilliant hues of the 

 plumage of the Humming-birds is derived from 

 some elements contained in the blood, and ela- 

 borated by the circulation, a theory we do not 

 quite understand, inasmuch as colour is the re- 

 sult of the reflection of some rays and the ab- 

 sorption of others, caused by the arrangement of 

 the molecules of any given body. He adds, 

 however, that the texture of the plumes plays 

 the principal part, in consequence of the manner 

 in which the rays of light traverse them, or are 

 reflected by the innumerable facets which a pro- 

 digious quantity of barbules or fibres present. 

 All the scaly feathers, he observes, which simu- 

 late velvet, the emerald, or the ruby, and which 

 one sees on the head and the throat of the Epi- 

 machi (as the grand Promerops of New G-uinea), 



