OCR Text |
Show 1882.] DR. H. GADOW ON THE COLOUR OF FEATHERS. 419 that through the combination of two different colours, as, for instance, yellow and blue, there would appear a third one, in this instance green. Thus, spectrum a, produced by the prism a, is partly covered by the spectrum ft, produced by prism b; the blue of ft covers the yellow of a. Now as red and green give white, and blue and yellow give green, we should in this case probably see only the colours red and orange, produced by prism a. Another circumstance, which might make this process very complicated, is implied in the consideration that the surface of the prismatic sheath is frequently uneven. How many different systems of prisms result from this arrangement, and how in the purple feathers of an AUthopyga the rays of light become broken by the surface-ridges into blue and violet, we are unable to explain. A third phenomenon, which needs explanation, is that some of the most gorgeously metallic feathers cannot be made to look black in position A or C. An example of this is the beautiful coppery-red to deep blue of a Jacamar. Under the microscope the compartments of the radii of such a feather are extremely convex, as in fig. 5, Plate XXVIII.; consequently there will be always some part of such a compartment which presents a vertical plane to the eye, and which therefore is always more or less in position B. Now to sum up. W e have to distinguish between several categories of colours in feathers. 1. Objective chemical colours directly produced by pigment. To these belong black, brown, red, orange, and yellow. 2. Objective structural colours. The feather may contain no pigment at all, and the colour be produced solely by a special structural arrangement of the feather-substance, for instance white, and frequently yellow; the latter if the surface is composed of very fine and narrow longitudinal ridges. Or the feather contains a yellow to brownish-black pigment, and the colour actually observed, as green, blue, and violet, is produced by a specially produced and particularly constructed transparent layer between the pigment and the surface. Of non-changing colours blue and violet are always structurally objective. Green seems to be only in a few cases the result of yellow pigment combined with blue surface-structure. In most cases it seems to be not a mixture of two colours, but due to yellow-pigment light being broken into green. A green pigment seems to be very exceptional. 3. Colours which change and which entirely depend on the position of the light and eye. They are produced by a transparent sheath, which acts like a prism. Any changing colour represented in the solar spectrum may be thus produced in feathers. The facts which I have laid down in this communication indicate the desirability, nay even the absolute necessity, of a standard method, not only ofdescribing but also of drawing birds and other animals which show metallic colours. These three standard positions, I venture to submit, should he the following: - Position A, in which the eye is placed between the bird and the li«-ht, the eye and light almost in a level with the planes to be examined. |