On a Periodic Structure in Many Insect Scales, and the Cause of Their Iridescent Colours

Ever since the distinction between the pigmentary and the structural colours of insects was recognised, there has been discussion as to the cause of the latter phenomenon, but no satisfactory explanation has yet been given. A great variety of theories has been suggested, but the very unsatisfactory and controversial state of the problem may be gathered from Prof. R. W . Wood’s statement in a recent paper,* to the effect that not only are the existing theories inadequate to explain the phenomena, but that these “ cannot be explained by any of the common laws of Optics with which we are familiar.” The theories that have been advanced to account for these metallic colours fall under the following headings :—• (1) The diffraction of light at the surface of a grooved structure or “ grating.” (2) The interference of light at the surfaces of “ thin films,” which may be either single or multiple. (3) The scattering of the shorter light-waves by bodies composed of particles smaller in diameter than the waves themselves, which allow the longer red waves to pass unhindered. The blue of the sea, snow, and sky is of this nature. It has sometimes been called “ blue due to scattering by small particles,” and that expression will be adopted in this paper.