272 THE METALLIC COLORS OF FEATHERS 



As has been stated before in this paper, these measurements are only approx- 

 imate estimates. The complex individual variations in the color-producing struc- 

 tures make attempts at greater precision of no value in a paper of this nature. 



We may sum up these observations as follows: (1) when the sum of the angles 

 of incidence and reflection is less than about 90° and the incident rays make an angle 

 of at least 48° with the plane of the feather, a brilliant green is obtained; (2) when 

 the sum of these angles is greater than 100° and less than 140°, purple effects are 

 obtained; (3) when the sum of the angles is greater than 140°, no metallic colors 

 appear. 



The refraction-prism hypothesis of Gadow ('82) requires that the angle or series 

 of angles just described be more than 180°, a physical impossibility here, for the fol- 

 lowing reasons: (1) either the incident or the reflected ("refracted") rays would 

 be intercepted by the contiguous barbules or by the barb; (2) metallic colors do 

 not appear unless this angle is more than 140°. "Refracted" rays that could pos- 

 sibly reach the eye would require a source for the incident rays ventral to the feather, 

 whereas the illumination is necessarily dorsal. 



5. The Nature of the Metallic Colors. — Even if the surface of the barbule were 

 covered with ridges or striae, it is inconceivable that they could produce these metallic- 

 color phenomena. Reflection gratings give nothing comparable to these effects; they 

 require special conditions of illumination with parallel rays of light passed through a 

 narrow slit, whereas the feather is iridescent in diffused daylight. Having satisfied 

 myself that the metallic colors under consideration are not reflecting-grating effects 

 produced by striae, ridges, knobs, or pits on the barbule surface, I was led to favor the 

 thin-plate hypothesis of Altum ('54, '54 a ) and Briicke ('61). The outer transparent 

 layer (Figs. 11, 12, tu.) is thin enough to produce interference colors, being not more 

 than three half green- wave lengths in thickness. Furthermore, it is very uniform 

 in thickness and has a comparatively smooth surface, as I have pointed out before 

 in this paper. The constant occurrence of pigment granules of a spherical shape 

 only in the barbules giving metallic colors, however, seemed too significant to war- 

 rant accepting the thin-plate hypothesis without qualification. 



At first I had regarded the function of the pigment simply that of an absorbing 

 background. I mounted a feather on a pad of paper and placed the mount on the 

 stage of a microscope. By means of a mirror, strong sunlight was reflected from 

 the side on the feather, and portions of single barbules were examined with a Leitz 

 No. 7 dry objective. The view obtained was exceedingly suggestive. There was 

 no uniform glow of diffracted light such as is ordinarily obtained from thin plates. 

 On the contrary, there appeared a large number of small glowing dots in a black field. 



