

[ 554 j 



LXI. On Metallic Colouring in Birds and Insects. 



By A. A. MlCHELSON*. 



[Plate IV.] 



"1T7ITH the exception of bodies which shine by their own 

 Y 1 light, the appearance of colour in natural objects is 

 due to some modification which they impart to the light which 

 illuminates them. In the great majority of cases this modi- 

 fication is caused by the absorption of part of the light which 

 falls on the object, and which, penetrating to a greater or 

 smaller depth beneath the surface, is reflected, and finally 

 reaches the eye. If the proportion of the various colours 

 constituting white light which is absorbed by the illu- 

 minated body is the same for each, the light which reaches 

 the eye has the same composition as before, and we say the 

 body itself is white ; but if this proportion be different, the 

 resulting light is coloured, and the colour of the body itself 

 corresponds to that colour or colour combination which is 

 least absorbed ; it is complementary to the colours which are 

 most stronglv absorbed. 



Thus the light from a green leaf in the sunshine, after 

 penetrating a short distance in the substance of the leaf, is 

 either transmitted or reflected to the eye. In its passage 

 through the substance of the leaf it has lost a considerable 

 part of the red light it originally contained, and the resulting 

 combination of the remaining colours produces the effect of 

 the complementary colour or green, as can readily be shown 

 by analysing the light into its elementary colours by a prism, 

 and comparing the resulting spectrum before and after the 

 reflexion from the leaf. 



The same explanation holds for all the paints, dyes, and 

 pigments in common use. 



These colour effects occur in such an immensely greater 

 proportion than all others combined, that the occasional 

 appearance of an exception is all the more striking. The 

 rainbow and the various forms of halo are almost the only 

 instances of prismatic colours which occur in nature. 



There remain only two other possible methods of producing 

 colour. 



A. Interference, including Diffraction. 



B. "Metallic" Reflexion. 



It has been abundantly proved that the usual " flat. 

 >rm " colouring, brilliant as 



* Communicated bv the Author. 



dead," " uniform " colouring, brilliant as this sometimes 



