358 TROPICAL NATURE 



and Mr. Sorby has detected no less than seven distinct 

 colouring matters in birds' eggs, several of which are chem- 

 ically related to those of blood and bile. The same colours 

 are often produced by quite different substances in different 

 groups, as shown by the red of the wing on the burnet-moth 

 changing to yellow with muriatic acid, while the red of the 

 red-admiral butterfly undergoes no such change. 



These pigmental colours have a different character in 

 animals according to their position in the integument. Fol- 

 lowing Dr. Hagen's classification, epidermal colours are those 

 which exist in the external chitinised skin of insects, in the 

 hairs of mammals, and, partially, in the feathers of birds. 

 They are often very deep and rich, and do not fade after 

 death. The hypodermal colours are those which are situated 

 in the inferior soft layer of the skin. These are often of 

 lighter and more vivid tints, and usually fade after death. 

 Many of the reds and yellows of butterflies and birds belong 

 to this class, as well as the intensely vivid hues of the naked 

 skin about the heads of many birds. These pigments some- 

 times exude through the pores, forming an evanescent bloom 

 on the surface. 



Interference colours are less frequent in the organic world. 

 They are caused in two ways : either by reflection from the 

 two surfaces of transparent films, as seen in the soap-bubble 

 and in thin films of oil on water ; or by fine striae which pro- 

 duce colours either by reflected or transmitted light, as seen 

 in mother-of-pearl and in finely-ruled metallic surfaces. In 

 both cases colour is produced by light of one wave-length 

 being neutralised, owing to one set of such waves being 

 retarded or shifted so as to be half a wave-length behind the 

 other set, as may be found explained in any treatise on 

 physical optics. The result is, that the complementary colour 

 of that neutralised is seen ; and, as the thickness of the film or 

 the fineness of the striae undergo slight changes, almost any 

 colour can be produced. This is believed to be the origin of 

 many of the glossy or metallic tints of insects, as well as 

 those of the feathers of some birds. The iridescent colours of 

 the wings of dragon-flies are caused by the superposition of 

 two or more transparent lamellae ; while the shining blue of 

 the purple-emperor and other butterflies, and the intensely 



