I. INTRODUCTION. 



During the summer of 1901 I began a study of variations in the finer structure 

 of pigeon feathers as a basis for a study of variations in the so-called physical colors 

 of pigeons. Among my material were a series of feathers from the necks of domestic 

 pigeons which exhibited metallic colors or iridescence. In a former paper (Strong, 

 : 02), I have given a preliminary statement of my results with these feathers. In 

 this paper I shall attempt to give a more complete account of my observations. 



I wish here to express my thanks to Professor C. 0. Whitman of the University 

 of Chicago for the material used and for numerous courtesies received in connection 

 with this work. 



The sides of the necks of the pigeons under consideration give more or less bril- 

 liant color effects by reflected daylight. If the observer stands between the source 

 of light and the bird, with consequent small angles of incidence and reflection, the 

 feathers appear metallic green. If, however, the bird is placed between the light 

 and the observer and the neck is viewed with rather large angles of incidence and 

 reflection, purplish effects are obtained. Various attempts have been made to ex- 

 plain such appearances in feathers. 



Altum ('54, '54 a ) and Briicke ('61) suggested that the metallic colors of feathers 

 are interference colors due to thin laminiB. Fatio ('66) did not consider the metal- 

 lic colors of feathers interference phenomena, but thought they are due to conditions 

 producing colored rings (" anneaux colores "), which, however, he did not describe. 

 He noted modifications in structure for metallic-colored feathers and an arrangement 

 of pigment granules in patches corresponding to the cells composing the fundaments 

 of the barbules. 



Gadow ('82) advanced a novel but inconceivable hypothesis. He attempted 

 to explain these phenomena as due to simple refraction, and he represented the trans- 

 parent portions of barbules * as acting like refracting prisms. 



The following statement with reference to the metallic colors of feathers was 

 made by Haecker ('90, p. 85): "Bei metallischen Federn sind die Fiedern II. Ordn. 

 die Trager des Metalglanzes. Bei denselben finden sich stets Langsrillen und Langs- 



* In the legend accompanying a figure, however, he calls the supposed refracting structure a "barb." I have 

 called attention to this possibly typographical error elsewhere (Strong, :02 a ). 



265 



