On Metallic Colouring in Birds and Insects. 555 



can be, e.g., in birds, butterflies, and flowers, finds its simple 

 explanation in the existence of pigment cells ; so that the 

 same cause (doubtless with many modifications) is here 

 effective as in the great majority of cases previously considered. 

 But the lively, variable "metallic''" glitter of burnished 

 copper or gold ; the reflexion from certain aniline dyes ; 

 the colours of certain pigeons, peacocks, humming-birds, as 

 well as a number of butterflies, beetles, and other insects, 

 requires another explanation. 



While cases under A occur occasionally in nature — for 

 example, in the colours of thin films, in the iridescence of 

 mother of pearl, and (as an accessory) in the colours of the 

 rainbow and of certain halos — they are so rare and so readily 

 distinguished from the true metallic colours that they may 

 be most conveniently treated as exceptions after the subject 

 of surface-colour has been considered. 



The designation " metallic " at once suggests that there 

 may be some common property of all these colours which is 

 typified by the metals themselves. But, as is well known, 

 the principal characteristic which distinguishes the metals 

 from all other substances in regard to their action on light, 

 is their extraordinary opacity. 



A very important consequence of such great opacity is 

 that light is practically prevented from entering the substance 

 at all, but is thrown back, thus giving the brilliant metallic 

 reflexion so characteristic of silver, gold, copper, <fec. In 

 fact, the distance to which light can penetrate in most metals 

 is only a small fraction of a light-wave ; so that a wave- 

 motion such as constitutes light, strictly speaking, cannot be 

 propagated at all. Again, as this opacity may be different 

 for different colours, some would be transmitted more freely 

 than others, so that the resulting transmitted light would be 

 coloured ; and the reflected light would be approximately 

 complementary to the transmitted colour. 



For most metals the difference is not very great ; so that 

 the reflected light, except in the case of gold and copper and 

 a few alloys, is nearly white. In the case of the aniline dyes, 

 however, there is a marked difference, as is clearly shown by 

 their absorption spectrum. In transmitted light, even a very 

 small thickness of fuchsine shows no yellow, green, or blue, 

 and gives as a resultant of the remaining colours a beautiful 

 crimson. The light which it reflects, however, is just this 

 yellow and green which it refuses to transmit, and it ac- 

 cordingly shimmers with a metallic golden green colour, 

 which changes when the surface is inclined, becoming full 



202 



