Figure 4 Iridiscence in Papilo palinurus. a, SEM of an iridescent scale showing its arrayof concavities, each with a section that exhibits the curved multilayering shown bytransmission electron micrography in b. This structure produces two simultaneousstructural colours c, yellow and blue . d, The blue annulus is created by a doublereflection from opposite and perpendicular concavity sides. d also schematicallyillustrates the way in which incident linearly polarized blue light has its e-vector (dottedlines) rotated by this double reflection. Bars, a, 15 mm; b, 1 mm; c, 6 mm