276 University of California Publications in Zoology 'Vol. 13 



be produced by the rami alone, or by the barhules 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 (pi. 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 th& 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 scattering the light 

 in the same manner as the rami of Asyndesmus (pi. 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. Wlien 

 crushed thej^ are rendered transparent and colorless, and show no 



