162 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
lic colors, iridescent phenomena, and lustre are called structural colors. 
According to Haecker, green is a structural color except for the single 
case of turacoverdin, a pigment described ky Krukenberg (’82). 
The production of structural colors has been variously explained as 
due to either (1) light-interference phenomena or (2) diffraction or dis- 
persion of light-rays. Except for white, however, a dark granular pig- 
ment (melanin) has always been found associated with such effects. 
Peculiar modifications in structure are associated with blue colors. 
Altum (’54* ) observed that feathers giving bright blues have the barbs 
isolated, i. e., not connected with each other by barbules. 
Haecker (90) considered as necessary for the production of blue: (1) 
a thickened unpigmented cortex, (2) a deposit of brown pigment in the 
medullary cells of the barb, and (3) the occurrence of more or less poly- 
gonal, porous-walled ‘ Schirmzellen.” 
I have examined blue feathers from the indigo bird (Passerina 
cyanea), the blue-bird (Sialia sialis), Pitta sordida, Pitta moluccensis, 
Cotinga cayana, and the blue-jay (Cyanocitta cristata). The brilliant 
blue feathers furnished by Pitta and Cotinga have the barbules rudi- 
mentary or of insignificant size where the color is most intense. The 
lateral diameter of the barb is also greater than in the more proximal 
and less brilliant portion. Such feathers never appear blue except 
when seen from above. Their ventral surface gives a dull brown color. 
The ‘“‘Schirmzellen” are conspicuously developed (Plate 2, Figs. 10-11, 
el’. med.). 
The cavities of the ordinary medullary cells have a thick peripheral 
layer of dark brown pigment. In Cotinga I found no ordinary medul- 
lary cells, but the ventral cortex was thickened and appeared black from 
a rich supply of pigment. 
Blue feathers from the blue-jay, blue-bird, and indigo bird show no 
‘‘Schirmzellen,” but there is a pigmentation of the central medullary 
cells (Plate 1, Figs. 7-8, med.) similar to that observed in the Pittas 
(Plate 2, Fig. 11). 
The distal portions of blue feathers from the blue-bird which I exam- 
ined gave a much more brilliant blue than the proximal portions. The 
transition from bright to dull blue was abrupt. With the aid of a mi- 
croscope, it could be seen that a light blue color of uniform intensity 
was given by the barbs in both proximal and distal portions. Where 
the feather appeared bright blue, the barbules were absent. A similar 
relation between brightness of color and the absence of barbules has 
been noticed by other writers for other birds. 
