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. 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 conspicuously.

3

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, "Oberflächen 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

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.

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

group of molecules may cause selective reflection. It seems clear that this property can not belong to individual molecules, at any rate in the case of the aniline dyes, for their solutions absorb impartially all the colors which are not transmitted, and it is only in the solid state that their peculiarities as regards reflected light become apparent; at the same time there is no change in the light transmitted whether the dye is in solution or a dry film.

Before entering in detail into the reasons which seem to point to interference rather than selective reflection as the origin of iridescent colors, some general remarks may be made on the character of the structures examined.

These structures have been either feathers of birds or the scales of insects. There are few orders of birds in which examples of iridescent coloring can not be found, but without doubt the humming birds are the most brilliant, although peacocks, trogons, and many others are not very far inferior. In the insect world the finest examples are to be found amongst butterflies and the day-flying moths of the genus Urania. Some beetles also are provided with vividly colored scales. These belong mostly to the weevils (which include the Brazilian diamond beetle).

Many other insects among the Diptera, Neuroptera, and Hymenoptera show brilliant metallic colors on their integuments, but these are not provided with scales, and in many cases the color fades more or less when the specimens become dry. These I have not examined. Feathers and scales, however, are remarkable for the permanence of their iridescent coloring, and it is to these only that the present observations apply.

Some of the peculiarities of the structures as regards change of color with the point of view depend on the shape of the surface on which the color-producing material lies. If the surfaces are flat or nearly flat, reflection takes place as from a looking glass, and the angle through which the specimen can be turned while still showing the characteristic color is small. Often, however, the surfaces are convex bosses or ridges, and then the angle of incidence and reflection is that contained between the direction of the incident light and the normal to the tangent plane at the point where reflection takes place, and is therefore to a great extent independent of the position in which the specimen is held, since there will always, within wide limits, be tangent planes to the convex surfaces which reflect the incident light in the line of sight. In these cases the colors might at first sight be taken as due to pigment, both on account of their comparatively low intensity and from the small change in tint and intensity which is produced by altering the inclination of the general surface to the direction of the illumination. The

low intensity is of course due to the small area of each convex surface which reflects light in any given direction.

In attempting to investigate the origin of the colors many methods were employed, the first and most obvious being to cut thin sections normal to the color-producing surface and then to examine them with the highest microscopic power available. If the colors are analogous to those of thin plates, it is clear from the high intensity of the reflected light that more than one pair of surfaces must cooperate in the reflection. In general the reflected light is not even approximately monochromatic, and this fact limits the number of surfaces which can be supposed to act, but if the surfaces are supposed to be separated by air and placed at the most favorable intervals their number need not exceed three or four to account for the observed intensity and tints.

The most favorable spacing for the successive layers is that their thickness and the intervals between them should be a multiple of the half wave-length of the mean ray, reckoned in the length of the waves within the material of the layer, and it was thought possible that the thin sections might show a laminated structure.

For the material of feathers and insects' scales, μ is somewhere about 1.5 or 1.6, so that the least thickness for the plates of refractive material would be of the order of one one-hundred-and-fifty-thousandth and the air intervals one one-hundred-thousandth of an inchboth beyond the resolving power of the microscope; but from the composition of the reflected light it seemed likely that the intervals might be two or three half wave-lengths, which would be readily seen as far as adequate separation of the images is concerned. In nearly all the sections examined bands of this order of thickness appeared with some forms of illumination, but it was impossible to be sure that they were not due to diffraction effects from parts of the section slightly out of focus.

There are many difficulties in preparing sections thin enough for the advantageous use of objectives with large angular aperture. When a section is to show a stratified structure its thickness should certainly not be greater than the distance between the successive strata, and may with advantage be much less. It was not difficult to cut sections about one twenty-thousandth of an inch thick, but this is three or four times too thick to show with certainty stratification whose pitch is one sixty-thousandth or less.

Occasionally, by accident, thinner sections (perhaps one fortythousandth) would be cut, and these showed apparent stratification most plainly, but in no case was the image free from the effect caused by some part of the thickness of the section being out of focus, and, in all probability, what appeared to be stratification was in reality a series