NOTE ON THE IRIDESCENT COLORS OF BIRDS AND 



INSECTS. 1 



[With 3 plates.] 



By A. Mallock, F. R. S. 



Considerable interest attaches to the origin of certain forms of 

 brilliant coloring which are of frequent occurrence in the animal 

 world, though hardly represented among plants. 2 The colors in 

 question are those which are not due to ordinary pigment, and 

 which change with the angle of incidence of the light. The most 

 brilliant examples are to be found amongst birds and insects. Fishes, 

 and a few reptiles, exhibit colors of the same kind, but not so con- 

 spicuously. # 



During the last 10 or 12 years I have examined some hundreds 

 of cases of this sort of color production, and quite recently Michelson 3 

 has published investigations on the same subject, and refers to a 

 somewhat similar paper by Walter, "Oberflachen und Schillerfarben," 

 dated 1895, of the existence of which I was not before aware. 



The conclusions of these authors are that the colors in question are, 

 in most cases, due to selective reflection from an intensely opaque 

 material, and, in some few, to diffraction from a finely striated 

 surface. Their reasons for adopting the hypothesis of selective 

 reflection rather than interference are the close similarities as regards 

 the reflection of polarized light found between the natural iridescent 

 colors and dry films of aniline dyes. 



In the present note I give some reasons for the belief that in the 

 majority of cases interference of some sort is the active cause, 

 although in others the possibility of selective reflection is not 

 excluded. The question really turns on the size of the "grain" 

 of the color-producing structure. Is it comparable with the wave 

 length of light or of molecular dimensions ? 



If the colors are due to interference, the first supposition must be 

 true; but if selective reflection is the agent, a comparatively small 



i Reprinted by permission from Proceedings of The Royal Society, London, Series A, vol.S5,No. A 582, 

 Nov. 30, 1911, pp. 598-605. (Received by the society Sept. 12; read Nov. 2, 1911.) 

 2 Some Lycopodiums exhibit traces of iridescent color. 

 s "Metallic coloring in birds and insects," Phil. Mag., April, 1911. 



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