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Show 416 DR. H. GADOW ON THE COLOUR OF FEATHERS. [May 2, the green appearance is the result of a mixture of the yellow-pigment colour and of a blue optical structural colour. However, this cannot well be always the case, since most green feathers do not show that peculiar structure which we invariably meet with in blue feathers. All the green feathers which I have examined show the following structure :-Generally a transparent smoothly surfaced sheath surrounds the rami and the radii, which are both green. Between this sheath and the invariably present yellowish, brownish, or pinkish . pigment one sees a system of ridges and fine pits. These ridges are shorter and less regular than those observed in yellow feathers, and the little pits are rather irregularly dispersed over the shaft and plane of the barbs and barbules. The more regular and parallel these furrows are, the more approaches the green colour to a yellowish tinge. As we know of no green feathers without any pigment, and always with such an irregularly ridged and furrowed surface-structure, we cannot say that this structure directly produces green, nor that it produces blue. W e must accept that they break the yellow light, issued from the yellow pigment, into green. Red feathers are frequently surrounded with a thick transparent sheath, for instance those of Rhamphastus ; but they have no peculiar or particular surface-structure, and the large wrinkles which we observe in them seem to be merely the result of a drying-up process of the horny feather-substance. In orange or orange-brown feathers, however, we frequently find a dark red pigment and yellow surface-structure. Explanation of subjective or metallic colours.-We speak of metallic colours if the feathers under reflected light appear with a metallic gloss, aud if their glossy colour changes into another one according to the position of our eye. If we look in a direction nearly parallel to the plane of the feather it will appear black. This can be done in two ways (fig. 3, p. 420) : first, with our eye between the object and the light, a position which I propose to call A ; secondly, with the object between the light and the eye, position C. By passing the eye from A to C, along the line indicated by the arrow, we notice the gradual appearance of all the various metallic colours which the feather is able to display. W e further observe that these colours do not appear at random, but, and this is of the greatest importance, that they begin with the colours nearest to the red side of the spectrum, and end with the violet. The position just intermediate between A and C is that in which we look vertically down upon the plane of the feather, with the object turned fully to the light; no matter, however, whether this position is produced by looking at the feather in the way as indicated by diagram B or D. This position we call B. In order to ascertain this fact, I have examined, under these three positions, about eighty birds of all orders, wherever metallic colours were present, and I did not find oue single exception to the rule. With the exception of two particular cases, which I shall explain later on, the metallic parts of all these birds look perfectly black in |
