COLOUR-PRODUCTION IN THE FEATHERS OF BIRDS. 221 



12. Colour-production in relation to the Coloured Feathers 

 of Birds. By A. Mallock, F.R.S., F.Z.S. 



[Received February 7, 1921 : Read February 22, 1921.] 



(Text-figures 1-4.) 



The brilliant colouring of many birds and the forms of the 

 feathers on which this colouring appears, present many points 

 of interest both in regard to the means by which the colon r-efi'ect 

 is produced, and to the processes of selection which have led to 

 their development. 



In the present note I touch only on the physical side of the 

 problems, and hope to give some idea of the many ways in which 

 colour may be produced by the action of white or composite light 

 on matter, and of the effect of the forms of various feathers in 

 modifying the appeai'ances which the more intimate structure of 

 the material produces on the incident light. 



If any object appears coloured when viewed in white light, it 

 shows that the matter of Avhich it is composed exercises some 

 selective action on the composite light falling on it, absorbing or 

 transmitting certain colours and reflecting or scattering the 

 remainder. This selection may be of two kinds : namely, a 

 relation between the periods of light of various wave-lengths and 

 the molecular periods of the matter on which it falls, or on a 

 relation between some distance or spacing in the structure of the 

 substance and the wave-lengths themselves. 



The first of these relations includes all pigment colours, and 

 the second those which are known as interference effects. 



In the following table I have given a list of all the types of 

 colour-production with which I am acquainted, and I believe 

 that these will cover every know T n case, although the actual 

 dynamics of'a large proportion are very doubtful, involving as 

 they do the dynamics and constitution of the molecule. 



Colour may be produced by : — 



I. 



A relation between the periods of waves of light and the 

 molecular periods of the colour-producing matter : 



Examples. 



Dispersion, 

 i.e. dependence of wave velocity on 

 wave-length. 



All dyes and pigments, which may be 

 transparent and scatter or transmit 

 waves of certain periods and absorb 

 the remainder. Or : — 



Reflect certain periods and transmit the 

 remainder. 



Prismatic Colours, Rainbows, etc. 



Powdered glass or other colourless trans- 

 parent material immersed in a fluid of 

 the same mean refraction index but 

 different dispersive power. 



The greater number of coloured solids 

 and fluids. 



Aniline colours in crystal or dry films, 

 and many other crystals. 



16* 



