I 3° 
Jeffries on the Colors of Feathers . 
Interference acts in several different ways, all of which are 
based on the same principle, and so films may be taken as an 
example. If a beam of light, xy (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 equal. The rest 
of the rays will pass through the film to the other surface, being 
slightly refracted in their course. Here part will be reflected, 
and being again refracted at the first surface, will emerge in a 
line wz' nearly coincident with yz , the balance passing out into 
the air. Now the waves composing the white light of two beams 
yz and wz' will run together and partially obliterate each other, 
after the manner of ripples on water. Accordingly certain waves 
will be obliterated, and since white light is due to the blending of 
waves of the different colors, the light reflected from the film will 
be that of the colors not interfered with, the waves thus oblit- 
erated depending upon their length and the thickness of the 
film traversed. So as we look at the film from different points 
the conditions vary, and with them the resultant color. 
Interference may also produce colored light by means of fine 
particles diffused through another substance, as milk in water, 
the particles in the air, and the like. Colored light produced in 
this way is known as opalescent, the transmitted light tending to 
the red end of the spectrum, and the reflected to the other por- 
tions. This result can be obtained by mixing black and white 
grains, an experiment which all have tried as school bovs, bv 
soaking chalk in ink, the result being a bluish color. 
Diffraction acts apparently by bending the light rays different 
amounts, and thus spreading out the spectrum. Explanations of 
the various 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 peculiarities of which 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 ( Sialia stalls). Pigments of various colors are known 
to occur in feathers, and have received special names, as turacin, 
zoonery thrin, zoofulvin, zooxanthin, zoochlorin, zoomelanin. 
These evenly distributed, as turacin, zoonerythrin, and zoofulvin, 
or in patches, as zoomelanin, impart their respective colors to the 
