268 THE METALLIC COLORS OF FEATHERS 



until we have the dorsal half of the barbule deep black by transmitted light except 

 for the narrow spaces where inter-cell boundaries occur. 



There is also a great change in the arrangement of the barbules. Instead of 

 lying with their broad sides at right angles to the plane of the feather, they are 

 turned so that one sees the flat side and not simply the dorsal margin (compare Figs. 

 3, 4). The dorsal margins then rest on the median region of the next more distal 

 barbules (Fig. 4) and the ventral sparsely pigmented halves are hidden from view. 

 By this arrangement, only the heavily pigmented portions of the barbules are visible 

 and a much greater amount of reflecting surface is obtained. Similar conditions 

 were noted by Fatio ('66) in the metallic-colored feathers of some other birds. There 

 is some variation in the sizes of these modified barbules, those near the distal end 

 of the barb being the shortest. Individual contiguous barbules also vary a little 

 both in size and form. They are also, as shown in Figures 2 and 4, not strictly 

 uniform in position, there being some variation in the amount of surface hidden from 

 view. 



In Figure 4 are shown unpigmented angular processes (brbc.) at the distal ends 

 of the barbules. As they make a considerable angle with the sides of the barbule 

 they are not noticeable in the lateral views of single barbules given in Figures 7 and 

 10, though Figure 7 shows a portion of such a process. They seem to be vestigial 

 barbicels. 



These barbules are somewhat irregularly concavo-convex, the convex side being 

 dorsal in Figure 4. In Figure 11 are shown sections of several barbules, the sections 

 from left to right being successively nearer to the proximal end of the barbule. It 

 will be readily seen that the concavity and convexity vary in different parts of the 

 barbule, and a comparison of several such groups of barbule sections shows that there 

 is considerable individual variation for different barbules in this respect. 



It will be noticed on examining the two most proximal barbules in Figure 4 that 

 the amount of unpigmented surface appears small as compared with that shown 

 in the lateral views of single barbules given in Figures 7 and 10. This is due to the 

 sharp convexity of the barbule near its middle. The heavily pigmented dorsal half 

 of the barbule makes a very small angle with the plane of the feather, whereas the 

 ventral sparsely pigmented half makes a much larger angle and dips more or less 

 sharply downward. (Compare Figs. 4 and 11.) 



The dorsal portions of the barbules are much thicker than the ventral (Fig. 11), 

 and the dorsal margins are rounded. There is a thin transparent layer of keratin 

 less than one micron in thickness which encloses the cell cavities (Figs. 11, 12, tu.). 

 This layer is very uniform in thickness and its surface is relatively smooth. The cell 



