366 M. V. Fatio on the Coloration of Feathers. 
coloration of the parts which are most exposed to them, readily 
explains why the dorsal surface of a feather is usually more highly 
coloured than its ventral or inferior surface. 
Besides the coloration seen by transmitted light and pro- 
duced by various pigments, which act the same part with light as 
any other coloured bodies, I explain the brilliancy of ordinary 
feathers, the coloration of enamelled feathers, and the varying 
metallic lustre of optical feathers by the following phenomena of 
interference. 
The development of the cortical substance increases the bril- 
liancy by multiplying the reflecting surfaces, and at the same time 
increasing their distance. The meeting of these rays, reflected 
at various distances, produces an effect nearly similar to that 
described by Dove * in some bodies. I should even be tempted 
to ascribe the production of the blue colour by the enamelled 
feathers to a phenomenon analogous to that by which the above 
learned physicist endeavours to explain the brilliancy and lustre 
of some bodies,—namely, to the superposition of a transparent 
reflecting layer (in my case slightly coloured) upon a base covered 
with dark-coloured designs. In fact, if I scrape away this external 
varnish at some point, the blue feather appears black or brown 
at this part. 
In the lustrous optical feathers a new complication is added to 
these first effects. It indeed recalls the designs and streaks of 
Dove, but seems nevertheless to approach more nearly to the 
phenomenon of the coloured rings. ‘This is a series of transverse 
lines, sometimes brilliant, sometimes obscure, corresponding with 
the strongly marked segmentation of the barbules, as may be 
easily ascertained by examining an optical feather with a low power 
under direct light. The effect of each barbule is added to that 
of the following one, and we always find a much more regular 
arrangement of the barbules in the feathers which have the 
strongest metallic lustre. 
We must not confound the coloration extravasated in powder, 
of which I have spoken above, with another coloration deposited 
in the same form, but from the outside, upon the feathers of some 
birds. In the latter case, itis by rubbing against foreign bodies, 
vegetable or mineral, that some species cover certain parts of 
their bodies with a tr uly external and more or less solid coloration. 
Nor must we confound the decolorization which takes place upon 
a living dividual with that which occurs slowly in our muse- 
ums. The decolorization in collections arises most frequently 
from a saponification of the coloured fatty matters, produced 
in course of time by air and light, as also from a disintegration 
* Abhandlungen der Akad. der Wiss. zu Berlin, 1855. 
