THE BUILDING OF A BIRD 17 



bow. 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 pig- 

 ments 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 when they 

 were moulting, " does it hurt birds to lose their feath- 

 ers, uncle ? " 



" It is probably not as comfortable as being nicely 

 dressed, and sometimes they seem quite miserable, espe- 

 cially if they shed old feathers faster than new ones 

 can grow to replace the lost ones. Some birds, like 

 Ducks, lose their wing-feathers all at once, and cannot 

 fly for quite a while. But Heart of Nature is kind to 

 his children, as a rule. Most birds shed their rowing 

 feathers one at a time in each wing, so that they never 

 lose their power of flight. Now this will do for wings, 

 tails, and feathers. Come ! what is the next thing 



