feathers which change from bluish green through red to yellowish 
green with increasing angle of incidence. Thickening of the 
color-producing films by swelling results in a color change toward 
a color listed in a series after the original color, for the order of 
the color is higher the thicker the film. Thinning by pressure 
or in fault bars results in a color of lower order (earlier in the se- 
ties) than the original color of the barbules. 
Bleaching does not destroy the colors, because there is no 
bright pigment to be destroyed, but only a structure; and the 
iridescence is uninjured unless the structure is destroyed. The 
metallic luster and brilliancy of iridescent feathers is due to the 
dark brown pigment in them, which serves as a dark background 
for the interference colors. The exact distribution of this dark 
pigment is not certain; but apparently no special distribution is 
necessary to give the metallic luster, provided the essential 
structure is present, for the white, faintly iridescent, pigeon 
feathers become brilliant and metallic when dyed with a brown 
dye, though a detailed study of sections of the dyed barbules 
shows a rather uniform distribution of the dye instead of segre¬ 
gation in one underlying layer. It is possible that a neutral- 
colored, transparent, thin film may serve to some extent as its 
own dark background, producing brilliant colors and a metallic 
luster, as does a colorless film on a dark background. As a 
matter of fact, we get brilliant colors with thin films of asphaltum 
varnish. 
The metallic colors can be duplicated very satisfactorily by 
putting a thin film of a colorless varnish over a dark pigeon’s 
feather, but the result is more garish than the real feather, be¬ 
cause the film spreads over the feather as a whole instead of over 
each individual barbule. The curvature of the barbules, with 
the resultant complexity of the surface of the feather, results in 
a softening and blending of the garish colors noticed in plane, 
thin films, because of the different angles at which different 
parts of the curved surface are viewed. This causes a mingling 
of the colors seen at normal and at other angles of incidence, 
giving added softness and richness of color. The slight local 
variations in the thickness of the color-producing film also 
contribute to this effect by causing a mottling of colors, which 
mingle and appear to the eye as the mean color of all the various 
hues. The color which we observe on the feather is thus the 
result of the mingling of a number of colors adjacent in Newton’s 
series, which produce the effect of a single, softer color. No 
one can deny that the colors of thin oil films on a pavement are 
garish. 
Variation in the thickness of the color-producing films ex¬ 
plains the different colors often observed on a single feather, 
either in the form of a pattern, as in the case of the peacock, or 
in gradations of color, such as are seen in the feathers of the 
starling, purple grackle, Lamprocolius phoenicopterus bispecularis, 
and other birds. Such iridescent feathers show bands of colors 
which remind one very much of the colors produced by oil on an 
asphalt pavement. For instance, some of the feathers of the 
starling are iridescent reddish purple at the base shading to 
( 3 ) 
