130 Jeffries on the Colore of Feathers. 



Interference acts in sc\cral dificrent ways, all of \\hich are 

 based on the same principle, and so films may be taken as an 

 example. If a l)eam of light. .\'V (figure i). is allowed to fall 

 on any thin plate, or film, part of the rays will be reflected in 

 the direction yz. the angles byx and ayz being ccpial. I'he rest 

 of the ravs will pass through the film to the other surface, being 

 slighth refractetl in their course. Here part will be reflected, 

 and being again refracted at the first surface, will emerge in a 

 line x{.'^' nearly coincident \yith yz^ the balance passing out into 

 the air. Now the wayes composing the white light of two beams 

 vz and ivz' will run together and partially obliterate each other, 

 after the manner of ripples on water. Accordingly certain wayes 

 will be obliterated, and since white light is due to the blending of 

 Ayayes of the different colors, the light reflected from tiie film w ill 

 be that of the colors not interfered with, the wayes thus oblit- 

 erated depending upon their length and the thickness of the 

 film trayersed. So as we look at the film from difierent points 

 the conditions yary. and \vith them the resultant color. 



Interference may also produce colored light b\' means of fine 

 particles difl'used through another substance, as milk in water, 

 the particles in the air, and the like. Colored light produced in 

 this way is knoyyn as opalescent, the transmitted light tending to 

 the red end of the spectrum, and the reflected to the other por- 

 tions. This residt can be obtained by mixing black and white 

 grains, an experiment which all haye tried as school l)o\s. b\ 

 soaking chalk in ink. the result being a bluish color. 



Diffraction acts apparently by bending the light rays ditlerent 

 amounts, and thus spreading out the spectrum. Explanations of 

 the yarious phenomena of this sort are difficult, and need not 

 be entered into here. 



Feathers are classed, according to their appearance, into ordi- 

 nary, metallic and iridescent, the ]:>eculiarities of \vhich are well 

 known and so need not delay us. 



The ordinary feathers are colored by simple pigments, by con- 

 trast of light and darkness and mechanically, as in the case of the 

 Bluebird (ySlalla sialis). Pigments of yarious colors are known 

 to occur in feathers, and ha\e receiyed special names, as turacin. 

 zoonerythrin. zoofuhin. zooxanthin, zoochlorin. zoomelanin. 

 These eyenly distritmted, as turacin. zooner\ thrin. and zoofuhin. 

 or in patches, as zoomelanin. impart their respectiye colors to the 



