276 Unwersity of California Publications in Zoology Vou. 18 
be produced by the rami alone, or by the barbules alone. Although 
the physical principles upon which the color production rests are 
probably very much the same in all cases, the mechanisms or surfaces 
for producing it vary to an astonishing extent in different kinds of 
birds; the same color is by no means always produced in the 
same way. 
As a rule, white is produced merely by the absence of pigment, 
the barbules being translucent, or semi-transparent, and producing 
a white color by the diffusion of light by means of the numerous 
edges and irregularities of surface of the vanules. In some cases, 
however, more complicated mechanisms are resorted to. In Lagopus 
the barbules from a white feather appear a peculiar fawn-gray color 
under the microscope by transmitted light, due to the presence of 
an infinite number of exceedingly small air bubbles in the substance 
of the barbules (pl. 24, fig. 47a). When the latter are broken, (i. e., 
the horny outer sheath rendered penetrable) and immersed in balsam, 
the latter substance, which has almost exactly the same refractive 
index as the substance of the feather, destroys the effect of these 
bubbles by filling in the air spaces, and it is rendered transparent. 
In many feathers which have conspicuously white shafts or barbs, 
all or a portion of the barb is filled with a mass of these minute 
bubbles, appearing under the microscope dark and opaque by trans- 
mitted light, but glistening white, like a minature snow bank, by 
reflected light. Such a phenomenon may be seen on the lower side 
of the rami of belly feathers of Asnydesmus. 
The silvery straw color found on the outer vanes of the sec- 
ondaries, wing coverts, and scapulars of Plotus anhinga is produced 
in an absolutely unique way. The proximal barbules and bases of 
the distal barbules are black, while the tips of the distals are highly 
modified, inflated, and without pigment, though seattering the light 
in the same manner as the rami of Asyndesmus (pl. 18, figs. 13c, e). 
Like the white rami of the latter, they are rendered transparent 
when pervaded by balsam. 
Yellow is sometimes produced by pigment alone, especially in 
such yellows as those of orioles and wood-warblers, and is then usu- 
ally produced by pigment in both rami and barbules. Many yellow 
feathers, e. g@., the straw yellow of the head and neck of Paradisea 
apoda, possess little if any pigment, and have their color produced 
by naked rami with longitudinal grooves, or irregular pits. When 
crushed they are rendered transparent and colorless, and show no 
