266 THE METALLIC COLORS OF FEATHERS 



streifen, welch Beugungserscheinungen hervorzurufen scheinen." In a subsequent 

 paper, however, Haecker und Meyer (:01) refer to the whole subject of metallic colors 

 as an unsolved problem. 



Newbigin ('98) has also called attention to modifications in structure associated 

 with metallic colors and to the presence of a considerable amount of dark pigment. 



II. METHODS AND MATERIAL. 



Most of my work has been done with neck feathers from the homer pigeon, but 

 metallic-colored feathers from other birds have been examined. I have depended 

 largely upon dissections and teased preparations of the dry feathers. Sections of 

 feather germs were also prepared, however, by the methods described by myself in 

 another paper (Strong, :02 a ). The sections of barbules shown in Figures 11 and 

 12 (PI. XX) were obtained from cross-sections of the feather germ near its distal end, 

 where conification was complete and the barbs were arranged approximately at right 

 angles to the plane of section. The barbules were consequently cut somewhat 

 obliquely, as can be seen by drawing a fine across the barbules in Figure 4 at right 

 angles to the long axis of the barb. 



For the measurement of the angles made by the incident and reflected rays of 

 light, the following simple apparatus was devised. An axis (PL XX, Fig. 15, d.) pass- 

 ing through the centre of a protractor (e.) at one end, was mounted in a pasteboard 

 box. The feather to be examined was fastened with its dorsal surface upward on 

 a piece of stiff black paper. This paper was glued on the axis. Various inclinations 

 of the plane of the feather could then be measured on the protractor by an indicator 

 (/.). Another indicator (g.) revolving about the axis as a centre but independent 

 in motion, was used in measuring the position of the eye. Parallel rays of sunlight 

 were secured by reflecting sunlight from the mirror (a.) through a slit (b.) in the side 

 of the dark chamber to the feather at c. I use the expression incident rays for rays 

 of sunlight coming from the slit (b.) to the feather at the position c. The course 

 of reflected rays, as the expression is used in this paper, was measured by means of 

 the indicator (g.). 



HI. OBSERVATIONS. 



i. Non-metallic Color Conditions. — In order to understand properly the modifi- 

 cations that accompany the production of metallic colors, we shall first consider the 

 structure and arrangement of those portiens of the feather that do not give metallic 

 colors. 



