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Show 1882.] DR. H. GADOW ON THE COLOUR OF FEATHERS. 417 position A; if we turn the bird to position B it will, let us say, green; and halfway between B and C this bird will assume a blue colour, which again passes into violet before appearing black again in position C. If the bird begins with bronzy red, it will change through golden green to pure green, then through bluish green to blue and violet. There is not a single feather which, if moved from B to A, changes from green to blue, in other words from the violet to the red end of the spectrum. Thus we are able to predict into which colour a bird can change if we know its colour in position B. Thus a blue feather can only become purple or violet ; a green one has more changes, and a golden-green one still more. It is, however, very rare that a feather changes through more than half of the spectrum ; a coppery-red feather will generally cease with green ; a violet feather cannot change at all, except into black, since beyond the violet there are no visible colours in the spectrum. Another important fact is that metallic feathers can appear in any colour which is represented in the spectrum, but not in any which, like brown or grey, are not spectral colours. All these circumstances induce me to explain the changeable metallic colours as prismatic; and in order to prove this we have to examine the feathers for their prismatic structure. In any metallic feather the metallic colour is confined to the radii which are entirely devoid of cilise, and consist of a series of variously shaped compartments which overlap one another like tiles of a roof. Fig. 1, Plate XXVIII., represents a barbule of a violet feather of Mthopyya; it consists of about fifteen compartments, each 0*03 mm. long and 0*03 m m . broad, and each forming a plane. Fig. 9 a, Plate XXVII., shows part of a barbule ofNectarinia famosa, brilliant green in position B, blue in C, black in A ; each compartment was 0*027 m m . long and 0*015 m m . broad. If turned on the edge and looked upon sideways, they look like fig. 9 b ; each compartment is convex-concave, with the convex side lying uppermost turned towards the light. In Jacamar, coppery red in positions B and A, green in C, they measure 0*040 by 0*018 m m . Every one of these compartments is surrounded by a transparent horny coat, the thickness of which varies from 0*00085 m m . (Sturnus) and 0*0015 m m . (Jacamar), 0*0012 (Mthopyga) to 0*0022 m m . ( Galbula tombacea). The surface of this coat is either perfectly smooth and polished, like in Nectarinia famosa, or, in violet feathers, contains very fine longitudinal ridges, or, as in Jacamar, it showed very fine and numerous little dots. Below this transparent and apparently colourless sheath lies brownish to black pigment evenly dispersed. Now, then, let us take a transverse section of such an arrangement (S S being the transparent sheath, P the pigment), and put it in a position that the light falls upon it from L (fig. 2, p. 418). A very small part of the orbit of a circle or any curve may be treated as a straight line ; we thus may regard this sheath as consisting of a number of small prisms. W e know that a prism in sucb a position, with the top or one edge directed upwards, breaks the light in such a way as to produce a spectrum on the side furthest from the light, with the |