NOTE ON THE IRIDESCENT COLORS OF BIRDS AND 
INSECTS.* 
[With 3 plates. ] 
By A. Mattock, F. R. S. 
Considerable interest attaches to the origin of certain forms of 
vrilliant 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 
brilhant 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 * 
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 refiection 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 
1 Reprinted by permission from Proceedings of The Royal Society, London, Series A, vol.85, 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. 
8 ‘Metallic coloring in birds and insects,’’ Phil. Mag., April, 1911. 
425 
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