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Show 1882.] DR. H. G A D O W O N T H E COLOUR OF FEATHERS. 409 The broad nuchal collar of orange of N. cornutus is altogether wanting, and there is no difference between the colour of this part of the back. W e observe that the brilliant blue of the primaries of N. cornutus is much dimmer in N. uvaensis; but this may result from our specimens being caged birds, and consequently not in such perfect plumage. In size and shape the two species are identical. 5. On the Colour of Feathers as affected by their Structure. By Dr. HANS GADOW. [Eeceived May 2, 1882.] (Plates XXVII. & XXVIII.) The colours which we perceive in the things surrounding us may result from various sources, according to which they may be divided into two classes. The FIRST CLASS consists of so-called objective, chemical, or absorption colours. Such colours do not change or vary under any position of the light or eye ; they receive their colour always from a colouring-matter. This may exist in the form of a solution or as pigment. Animal objective colours are mostly due to pigment. The S E C O N D CLASS has been variously described as subjective, physical, or structural colours. They are the result of reflected or broken light. This may take place in various ways : - 1. Total reflection of light, e.g. the gloss on a polished surface. 2. The light may be broken infinitely often and be totally reflected. The result of this is white. 3. Diffraction by a prism. 4. Interference of colours. By this theory are explained the colours of extremely thin transparent plates, and those which are produced by a system of narrow ridges, e.g. iridescence of mother-of-pearl, the blue-heat stage of steel, and the colour of soap-bubbles. As I have not, in this communication, entered into any general history of the colours of birds' feathers, but have confined myself to the effects of structure, it appears unnecessary to give any detailed critical account of the work of m y predecessors, which should only make part of an elaborate and complete essay. The titles of the more important papers, however, are given in the footnote1. 1 P>. Altum, " Ueber die Farben der Vogelfedern ira Allgemeinen, und iiber das Scbillern insbesondere," Naumannia, 1854, p. 293. B. Altum, " Ueber den Bau der Federn als Grund ihre Farbung," Journ. f. Orn. 1854, pp. xix-xxxv. A. Bogdanow, " Note sur le pigment des plumes des Oiseaux," Bullet, de la Soc. d. Naturalistes de Moscou, 1856, p. 458. V. Fatio, "Des divers modifications dans les formes et la coloration des plumes," Mem. Soc. pbys. hist. nat. Geneve, xviii. (1866), p. 249, plates. K~. W . Krukenberg, ''Vergleich. physio! og. Studien,' ser. 1. pt. v., ser. 2. pt. i. [See also a further contribution (ser. 2. pt. ii.) which has only come into mv hands as the revise is leaving them.-H. G., July 25th.] 28* |