FROM THE SIDES OF THE NECK OF THE DOMESTIC PIGEON. 275 



9. Green is obtained from these feathers when the angle made by the incident 

 and reflected rays is less than about 90° and the incident rays make an angle of at 

 least 48° with the plane of the feather. Purple effects appear when the angle of 

 incident and reflected rays is greater than 100° and less than 140°, the incident rays 

 making an angle of at least 10° with the plane of the feather. At angles greater than 

 140° non-metallic colors appear. 



10. The refraction-prism hypothesis of Gadow ('82) is untenable, for it requires 

 that the incident rays make an angle of more than 180° with the reflected ("refracted") 

 rays — a physical impossibility in such a structure as the feather where the illumina- 

 tion and color effects are necessarily dorsal. 



11. The metallic colors of these feathers are probably thin-plate interference 

 colors or Newton's-rings effects which are produced where spherical pigment granules 

 come in contact with the outer transparent layer. The pigment also has the very 

 important function of absorbing light not reflected to the eye as metallic color. The 

 colors seen without a microscope are mixtures of colors from innumerable small points. 



V. BIBLIOGRAPHY. 



Altum, B. 



'54. Ueber den Bau der Federn als Grund ihrer Farbung. Jour. f. Ornith., Jahrg. 2, pp. xix-xxxv. 



Altum, B. 



'54 a . Ueber die Farben der Vogelfedern im Allgemeinen, iiber das Schillern insbesondere. 'Naumannia, 

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Brilcke, E. 



'61. Ueber den Metallglanz. Sitzungsb. Akad. Wiss. Wien., math.-naturw. CI., Bd. 43, Abt. 2, pp. 177-192. 



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'66. Des diverses modifications dans les Formes et la Coloration des Plumes. Mem. Soc. Phys. et Hist. Nat. 

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Haecker, V., und Meyer, G. 



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'98. Colour in Nature. A study in Biology. London, 8°, xii + 344 pp. 



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'99. On the Transmission of Light through an Atmosphere containing Small Particles in Suspension, and 

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