COLOURS OF ANIMALS. 185 



world. They are caused in two ways : either by reflec- 

 tion from the two surfaces of transparent films, as seen 

 in the soap-bubble and in thin films of oil on water ; or 

 by fine strife which produce 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 caused 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 metallic colours of humming-birds, are 

 probably due to fine striae. 



Colour a Normal Product of Organization. This 

 outline sketch of the nature of colour in the animal 

 world, however imperfect, will at least serve to show us 

 how numerous and varied are the causes which perpetually 

 tend to the production of colour in animal tissues. If 

 we consider, that in order to produce white, all the rays 

 which fall upon an object must be reflected in the same 

 proportions as they exist in solar light whereas, if rays 

 of any one or more kinds are absorbed or neutralised, the 

 resultant reflected light will be coloured ; and that this 

 colour may be infinitely varied according to the propor- 



