274 THE METALLIC COLORS OF FEATHERS 



IV. SUMMARY. 



1. The metallic colors or iridescence of the sides of the neck of gray domestic 

 pigeons are confined to the dorsal surfaces of the distal portions of the feathers. 

 Other parts of the feather give dull browns and grays. 



2. Individual barbs have more or less of their proximal portions in the non- 

 metallic region. The transition from metallic colors to non-metallic colors is abrupt, 

 and there is a correlated modification in the structure and pigmentation of the 

 barbules. 



3. The barbules of the proximal, non-metallic-colored region are of the typical 

 form for body coverts. In the distal, metallic-colored region the barbules are turned 

 so that one side faces upward. Barbicels and attenuated portions are absent. A 

 much greater reflecting surface is obtained by this arrangement. 



4. The barbules giving metallic colors are much more heavily pigmented in 

 their dorsal halves than those with non-metallic colors. The ventral halves have 

 very little pigment, but they are concealed in a dorsal view by overlapping adjacent 

 barbules. 



5. Individual metallic-colored barbules are irregularly concavo-convex, the 

 convex side being dorsal. The dorsal halves are much thicker than the ventral. 

 There is an outer transparent wall of keratin which encloses cell-cavities filled with 

 pigment. This layer is quite uniform in thickness and less than one micron thick. 



6. The pigment of the metallic-colored barbules is in the form of spherical granules 

 of melanin, whereas the non-metallic-colored barbules have typical rod-shaped 

 granules imbedded in irregular masses in the horn substance of the barbule. In the 

 ontogeny of the feather, typical pigment-cells or chromatophores are formed which 

 distribute melanin pigment-rods to the barbule cells in the usual manner. In those 

 cells composing the fundaments of metallic-colored barbules, however, these pig- 

 ment-rods are metamorphosed into spherical granules. 



7. I have observed no striae, ridges, knobs, or pits on the outer surface of the 

 barbule sufficiently numerous or uniform enough to produce the observed colors 

 by diffraction. 



8. The structures producing metallic colors have many complex variations in 

 surface. Individual barbules and the barb itself act as screens for portions of 

 adjacent barbules when the angles of incidence and reflection are large, thereby 

 reducing the total amount of surface producing metallic colors. 



