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Wetmore, Alexander, 1886-, et al. / Warm-blooded vertebrates

Chapter III: color and its arrangement,   pp. 25-38 PDF (5.6 MB)

Page 27

structure whose arrangement serves so to refract the light
as to break it up into brighter colors.
Structural colors may be divided into two classes, me-
tallic and nonmetallic. The metallic colors, characterized
by a sheen like the reflection from polished metal, are
produced by an abundance of dark pigment overlaid by a
dark sheath in the tiny divisions of the feather called the
barbules. In some types the metallic light is reflected
from a highly polished surface; in others, well illustrated
by the green of the sacred trogon or quetzal of Central
America, the light is broken up by minute lines on or
under the surface of the overlying sheath. In metallic
reflections the color varies with the angle of the light
under which it is examined, changing with the position of
the object relative to the eye of the observer. To ascertain
the true shade of the underlying pigment in such feathers,
hold them so that light shines through, or pound them to
destroy the reflecting surface, when the pigment will show
in its true color.
The nonmetallic structural colors are very common.
White feathers-where no pigment whatever is present,
the white appearance being due to a refraction of the light
in the barbules-illustrate the simplest of them. Examined
closely under the microscope, the barbules are seen to be
minute processes, flattened, and entirely colorless. If im-
mersed in balsam, which destroys their refractive power,
they become transparent and the white disappears. In
the shafts and barbs, the larger divisions of the feather,
the horny substance, called keratin, that composes the
outer sheath is made up of numerous elongated cells,
which, though colorless themselves, reflect light in such a
manner that they appear white. These cells are im-
permeable and still show white when immersed in fluid.
Blue is an excellent example of the nonmetallic structural
group of colors in birds, as it is of common occurrence and
never appears as a true pigment. A typical blue feather
from a blue jay, in which the barbs and barbules carry
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