134 Jeffries on the Colors of Feathers. 



Here it is of interest to note that the barbs of the brown female 

 Indigo-bird difier but sHghtly from the bright blue barbs of the 

 male. In the female the pigment is more difluse, and the outer 

 horny coat is thicker and less dense and lustrous. 



The above feathers with their smooth outer coat are connected 

 with true iridescent feathers by an intermediate group. I refer to 

 the highly-colored blue and green feathers of sucli birds as Chlo- 

 rophanes atrocr/status (Fig. 2) and Ccereba hicida. In these the 

 ends of the barbs are enlarged and the barbules reduced to a min- 

 imum, after the manner of the Woodpeckers; unlike them, how- 

 ever, the surface is rough, each cell being rounded out. When 

 examined under a microscope such barbs appear as if covered 

 with a mosaic of gems. Sections show, whatever may be the 

 shape of the barb, that the walls of the iridescent parts are ex- 

 tremely thin, so thin that exact measurements cannot be made wath 

 the instruments at my disposal. The thickness got when reduced 

 to fractions of an inch, is approximately totj^txtj^ of an inch, a 

 film sufficiently thin for all purposes of interference. Many of 

 these feathers when magnified show that the color is not uniform, 

 but that all the colors contribute their quota to the final color. 

 The figure of a section of a barb of Chlorophanes atrocristatits 

 will give some idea of such a feather. In this case the final color 

 seems to be the result of mixing the light reflected from the dark 

 fin(\ with that from the yellow triangular part. 



W^e now naturally come to the true iridescent feathers, of 

 which the Peacock may be taken as an example. The irides- 

 cent barbules are made up of flat, wonderfully thin cells, ananged 

 end to end. as shown in figure 5. When examined with trans- 

 mitted light, they are seen to be films full of a brownish pigment 

 more or less evenly dispersed through the mass. Wlien cut in 

 sections and looked at on edge they resemble, even under quite 

 high powers, the edge of a piece of paper. Here we have the 

 most admirable contrivance for the production of iridescent 

 light, the plates being fully thin enough, and all white light 

 which may get through the walls being taken up by the brown 

 pigment within. All the parts of the eye are constructed on the 

 same plan, and only provided with brownish pigments, hence the 

 color must be due to variations in the thickness. Here it is well 

 to notice that the colors are quite constant. 



