T 34 
Jeffries on the Colors of Feathers . 
Here it is of interest to note that the barbs of the brown female 
Indigo-bird differ but slightly from the bright blue barbs of the 
male. In the female the pigment is more diffuse, and the outer 
horny coat is thicker and less dense and lustrous. 
The above feathers with their smooth outer coat are connected 
with true iridescent feathers by an intermediate group. I refer to 
the highly-colored blue and green feathers of such birds as Chlo- 
rophanes atrocristatus (Fig. 2) and Coereba htcida. In these the 
ends of the barbs are enlarged and the barbules reduced to a min- 
imum, after the manner of the Woodpeckers ; unlike them, how- 
ever, the surface is rough, each cell being rounded out. When 
examined under a microscope such barbs appear as if covered 
with a mosaic of gems. Sections show, whatever may be the 
shape of the barb, that the walls of the iridescent parts are ex- 
tremely thin, so thin that exact measurements cannot be made with 
the instruments at my disposal. The thickness got when reduced 
to fractions of an inch, is approximately T (H)4 fff of an inch, a 
film sufficiently thin for all purposes of interference. Many of 
these feathers when magnified show that the color is not uniform, 
but that all the colors contribute their quota to the final color. 
The figure of a section of a barb of Chlorophanes atrocristatus 
will give some idea of such a feather. In this case the final color 
seems to be the result of mixing the light reflected from the dark 
end with that from the yellow triangular part. 
We now naturally come to the true iridescent feathers, of 
which the Peacock may be taken as an example. The irides- 
cent barbules are made up of flat, wonderfully thin cells, arranged 
end to end, as shown in figure 5. When examined with trans- 
mitted light, they are seen to be films full of a brownish pigment 
more or less evenly dispersed through the mass. When cut in 
sections and looked at on edge they resemble, even under quite 
high powers, the edge of a piece of paper. Here we have the 
most admirable contrivance for the production of iridescent 
light, the plates being fully thin enough, and all white light 
which may get through the walls being taken up by the brown 
pigment within. All the parts of the eye are constructed on the 
same plan, and only provided with brownish pigments, hence the 
color must be due to variations in the thickness. Here it is well 
to notice that the colors are quite constant. 
