180 THE FUNDAMENTAL ASPECT OF COLORATION 



phenomenon, so well exhibited in the blue of the sky, and in other 

 common features of Nature, was first explained by Tindall in 1869, 

 when he showed that finely divided particles of a transparent 

 substance possess the property of scattering light. If white light 

 penetrates a system composed of minute particles, immersed in a 

 medium of different refractive index to their own, the scattered 

 light will be blue, provided the diameter of such particles is small 

 as compared with the wavelengths of light. Since such a system 

 promotes the scattering of the short waves, the light is blue, while 

 the longer red waves pass unhindered. 



A number of investigators have regarded that blue produced 

 in this manner contributes to the phenomenon of iridescence, 

 especially in cases unexplainable by any of the more usual laws 

 of optics. Recent research, however, has revealed very few 

 cases that can be interpreted on this basis, and most blues are 

 attributable to other causes. According to Mason (1926), the 

 bluish " bloom " or efflorescence present on the bodies of certain 

 dragon-flies, and less frequently on the wings, consists of particles 

 in a fine state of subdivision. When wetted with a liquid of 

 refractive index near 1-5 the particles become invisible and the 

 dark integument, which they overlie, is revealed. Upon drying, 

 the original appearance is restored. In the few cases investigated 

 by him it was found that the bluish " powder " or efflorescence 

 satisfied in a general way the main criteria required when light is 

 scattered in the manner indicated. 



Selective Reflection. Onslow devoted considerable attention to 

 the highly metallic colours exhibited by many scaleless beetles 

 and other insects. He was concerned with the question as to 

 whether the colour is a surface effect or one due to interference. 

 In his experiments with certain highly metallic beetles he ascer- 

 tained that the colours do not disappear under pressure : on 

 immersion in highly refractive fluids the reflected colours still 

 persist : the transmitted colours, so far as they can be seen in 

 the thin surface layer, also persist : the colour-producing layer 

 appears to be so near the surface that no room is available for a 

 suitable structure that might produce the colours observed : the 

 colour changes, on reducing the thickness of the metallic layer by 



