FROM THE SIDES OF THE NECK OF THE DOMESTIC PIGEON. 273 



All the colors of the spectrum were visible, especially towards the margin, where the 

 reds were in excess. Over the broad surface of the barbule the luminous dots * were 

 yellowish green to pure green. 



It at once occurred to me that there was a direct connection between the metallic 

 colors of the feathers under consideration and the pigment granules. The size and 

 distribution of the luminous dots corresponded with the size and arrangement of 

 the spherical pigment granules lying next to the transparent outer wall. 



I had often observed the upper portions of these granules outlined against the 

 transparent enclosing layer, along the dorsal margin (Figs. 12, 13). In this view 

 by reflected sunlight minute beads of either white, red, or yellow light were 

 outlined just where I had so often seen the dark forms of the pigment spheres by trans- 

 mitted light. It occurred to me at first that the pigment spheres were producing 

 diffraction effects such as the well-known interference colors of small particles 

 described by Tyndall ('69), Rayleigh ('99), and others. It seems more likely that 

 we have thin-plate interference or Newton's-rings effects where each pigment granule 

 touches the thin, outer transparent layer. The pigment granules act as a reflecting 

 mirror. Often they lie a little above the level of the outer surfaces of surrounding 

 pigment granules, and in such cases they are easily seen to be the centre of light dis- 

 persion. When small fragments of a barbule are obtained by crushing, the relation 

 in position of the color phenomena to individual pigment granules is seen to advantage. 

 The pigment also plays a very important role in absorbing light not reflected to the 

 eye as metallic color. Where the pigment is absent or sparsely distributed, there is 

 a flood of white light which destroys metallic color effects if they occur at all. It is 

 for this reason that the ventral side of the feather is not iridescent. The spherical 

 pigment granules also present the phenomena of "anomalous dispersion." By trans- 

 mitted light they are dark brown, almost black; with reflected light, however, they 

 are a light metallic or yellowish brown. In strong, reflected sunlight, they appear 

 as brilliantly glistening bodies as is characteristic of bodies of such small size. 



The colors observed with the naked eye are mixtures of light rays coming from 

 innumerable small points. Individual cells of the barbule vary in the color given for 

 a single position of the feather with reference to the source of light and the eye, but 

 the color is fairly uniform over the surface of a single cell. 



* When the barbules are immersed in cedar oil and xylol, fluids having approximately the same index of refrac- 

 tion as keratin, these appearances disappear to a greater or less extent. 



