416 DR. H. GADOW ON THE COLOUR OF FEATHERS. [May 2, 
the green appearance is the result of a mixture of the yellow-pig- 
ment 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 sur- 
rounds 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 pig- 
ment, 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. We 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, how- 
ever, 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, and if their glossy colour changes into another one 
according to the position of our eye. If we look ina 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. We 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 inter- 
mediate 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 one 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 
