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[Philosophical Magazine, Vol. XXXVH. pp. 98—111, 1919.]
IT is singular that the explanation of some of the most striking and beautiful of optical phenomena should be still matters of controversy. I allude to the brilliant colours displayed by many birds (e.g. hummingbirds), butterflies, and beetles, colours which vary greatly with the incidence of the light, and so cannot well be referred to the ordinary operation of dyes. In an early paper*, being occupied at the time with the remarkable coloured reflexions from certain crystals of chlorate of potash described by Stokes, and which I attributed to a periodic twinningf, I accepted, perhaps too hastily, the view generally current among naturalists that these colours were " structure-colours," more or less like those of thin plates, as in the soap-bubble. Among the supporters of this viewj in more recent times maybe especially mentioned Poulton and Hodgkinson. In Poulton's paper§ the main purpose was to examine the history of the very remarkable connexion between the metallic colours of certain pupse (especially Vanessa urticce) and the character of the light to which the larvae are exposed before pupation. In a passage describing the metallic colour itself he remarks:
"The Nature of Effects Produced.—The gilded appearance is one of the most metal-like appearances in any non-metallic substance. The optical explanation has never been understood. It has, however, been long known that it depends upon the cuticle, and needs the presence of moisture, and that it can be renewed when the dry cuticle is moistened. Hence it can be preserved for any time in spirit. If a piece of dry cuticle is moistened on its upper surface the colour is not renewed, but almost instantly follows the application of spirit to the lower surface. Sections of the cuticle resemble those of Papilio' machaon described in a previous paper (Roy. Soc. Proc, Vol. XXXVIIT. p. 279, 1885), and show an upper thin layer and a lower, much
* Phil. Mag. Vol. xxiv. p. 145 (1887); Scientific Papers, Vol. in. p. 13, see footnote. t Phil. Mag. Vol. xxvi. p. 256 (1888) ; Scientific Papers, Vol. in. p. 204. £ Distinctly suggested by Hooke in his Micrographia (1665). § Roy. Soc. Proc. Vol. xjoii. p. 94 (1887).riangles FBG, DAE, and every point,     "°      D or rather every infinitely small region of given                   •Plg' 3*