"What makes all the different colors of birds, Doctor?" asked Rap.

"That is a hard question to answer. It is natural for birds to have
particular colors, just as some people have black eyes and hair, while
others have blue eyes and yellow hair. But I can tell you one thing
about that. Look at this Sparrow. All the colors it shows are _in_ the
feathers, whose various markings are due to certain substances called
'pigments,' which filter into the feathers, and there set in various
patterns. The feathers are painted inside by Nature, and the colors show
through. You see none of these colors are shiny like polished metal. But
I could show you some birds whose plumage glitters with all the hues of
the rainbow. That glittering is called 'iridescence.' It does not
depend upon any pigment in the substance of the feathers, but upon the
way the light strikes them. It is the same with the beautiful tints we
see on a soap-bubble. The film of water itself is colorless, but it
becomes iridescent. You might divide all the colors of birds into two
classes--those that depend upon pigments in the feathers, and those that
depend upon the play of light on the feathers."

"That's pretty hard to remember," said Nat; "but I know how a
soap-bubble looks, though I never saw any birds look that way. Please
show us one."

"I will show you two," answered the Doctor, who then went to his glass
case, and took out a Wild Pigeon and a Hummingbird. "Look at the shining
tints on the neck of this Pigeon, and see how the throat of this
Hummingbird glitters when I turn it to the light."

"That's the prettiest color I ever saw," said Nat, "and I can remember
about it now. But," he added, thinking of the way he had seen hens mope