Jeffries on the Colors of Feathers. 
x 33 
Blue colors seem to be accidental, that is, the result of other 
causes than pigments. Not only have all efforts to extract the 
pigments failed, but blue feathers appear gray when examined by 
transmitted light. Again, no blue can be found in transverse sec- 
tions of blue feather parts. This method of studying the colors 
of feathers is worthy of more extended use than it has yet had. 
Bv this means all physical effects of the outer coat ai\e avoided, 
and the exact position- of the pigments can be seen. Sections 
are quickly prepared by fastening the feather on to a piece of 
pith with collodion, and mounting sections pith and all. The 
pith keeps the sections on end, a result otherwise difficult to 
obtain. 
Gray-blues, such as those seen in De?idrceca ccerulescens, are 
due to opalescence. The feather is full of fine granules of black 
or darkish pigment, which in a manner already described produces 
a blue color. 
Brilliant blues, as those shown by Sialia sialis , Cyanospiza 
cyanea , Coereha lucida , and the like, do not seem to be suscep- 
tible of -a like explanation. The color is too intense and pure to 
be produced in such a small space by opalescence. So most 
authors have simply ascribed it to some other form of interfer- 
ence, as a thin outer plate, which would seem on examination to 
be the true cause. Figure 2, drawn from a section of a Bluebird’s 
barb enlarged about one thousand diameters, will give an idea of 
the structure found in such cases. The central cells are full of 
some dark pigment, probably zoomelanin, while the surface is 
bounded by a transparent layer of horn varying from ^ to 
Tutyi ) it °f an bich in thickness. Thus we have a contrivance not 
ill adapted to the production of interference colors, the black 
pigment absorbing all rays which are not reflected by the horn 
coat on the outside. Yet there are decided difficulties in this 
view. Thin as it is, the outer horn coat is thick compared to the 
length of light waves, and again the blue color is constant. How- 
ever, in spite of these objections, the color must be ascribed to 
the action of the outer coat of cells. The structure of other 
bright blue feathers is much the same, though differences in 
minutiae exist. Thus the outer layer of cells, the external walls 
of which form the outer coat of the barb, are devoid of pigment 
in the Blue Jay. (Fig. 3.) 
