M. V. Fatio on the Coloration of Feathers. 365 
In the mized feathers the barbules are persistent, because the 
cortical substance is equally distributed in the two axes. In 
them the latent coloration is concentrated principally in the barbs, 
and these possess ready communication with the barbules. These 
second feathers, like the ordinary feathers, contain various pig- 
ments and sometimes present much brightness of colour; but 
they never possess reflections, and rarely so much brilliancy as 
the preceding ones. 
In the optical feathers it is the barbule alone that can develope 
itself. It swells enormously, and very often swallows up in its 
mass its lateral hooklets, when it acquires a cylindrical or pris- 
matic form. Sometimes in spring it may actually measure three 
or four times as much in diameter as in the previous autumn. 
These feathers always contain brown pigments, and the sepa- 
rative septa of the superposed segments which compose each 
barbule are in them much stronger than in any other feathers. 
These third feathers present themselves under all forms, and of 
all the colours of the spectrum, but always with metallic reflec- 
tions. 
Lastly, in the enamelled feathers, it 1s again the barb that is 
developed and throws off the barbule, although these are never- 
theless optical feathers, invariably furnished with dark pigments. 
In them the cortical substance has not been developed into fibres,— 
on the contrary, it presents itself in the form of large polygonal 
cells with the nuclei strongly coloured brown. On the dorsal 
surface of the barbs the cells, much less strongly coloured, are 
elongated and vertical, and form as it were a transparent external 
varnish of greater or less thickness. To thisfourth kind belong 
some green feathers, and especially all the blue feathers without 
metallic lustre. 
The observation of these four divisions allows us to establish 
the following general laws :— 
1. Of two successive axes, one is always developed at the expense 
of the other. 
2. In the ordinary feathers, properly so called, the secondary 
axis predominates over the tertiary. 
3. In the optical feathers, properly so called, the tertiary axis, 
on the contrary, predominates over the secondary. 
A. The mixed feathers present a mean condition. 
B. The enamelled feathers are optical in their pigmentation, and 
ordinary tn their development. 
The mived character is very often met with in a feather together 
with a development of another kind; but no feather can be at 
once ordinary and optical, any more than ordinary and enamelled 
or enamelled and optical. 
The influence of humidity and light upon the development and 
