8 THE COLOUKS OF ANIMALS 



on the various parts of the bubble, because the thin- 

 ness also varies. A bubble of melted glass may be 

 blown thin enough to produce the same effects, which 

 are also well seen when a thin layer of air is enclosed 

 between two sheets of glass or between the plates of 

 some crystals, or when a drop of oil is allowed to 

 spread out into a thin film on the surface of water. 

 When a substance has a laminated structure, and 

 sufficiently thin films are enclosed between the laminae, 

 very marked effects are seen. Thus the metallic 

 appearance of the laminated flakes which are formed 

 on the surface of glass which has been long buried 

 in the earth, is accounted for. If these brilliant 

 flakes are wetted the colour fades away, because the 

 thin films of air between the laminas are displaced by 

 water, with a refractive power much nearer to that of 

 the glass, and the amount of reflected light is there- 

 fore diminished. 



Interference colours due to thin films are certainly 

 very important among animals, but the extent to 

 which they occur is imperfectly known. The irides- 

 cent colours of many beetles' wings are probably due 

 to thin films of air included between layers of a horny 

 consistence Such colours are unaltered in dried 

 specimens. In other cases the chinks between the 

 layers are kept open by films of less powerfully re- 

 fractive liquids. When the tissue becomes dry the 

 films evaporate and the colour disappears. We 

 must suppose that the denser layers come together, 



