282 FIRST LESSONS IN ZOOLOGY 



they may contain onl)' air, in which case the)' are trans- 

 parent or whitisli under the microscope, or they may 

 hold small granules of pigment, a colored substance 

 which makes them brown or yellowish or reddish or 

 blackish. 



Some butterflies have blue or green or purple colors, 

 irridescent and changeable, on their wings. The common 



Fig. 219. — Ovvl-butterlly (t'l/A^'c), undcT sidr (Two-thinls natural sizr ; 

 ]ili. .toLjrapli by 11m- auUioi-,) 



little "blues " have the upper side of the whole wing 

 metallic blue. Examine under the microscope some 

 scales from one of these blue wings, or from a blue or 

 greenish iridescent spot on any butterfly's wing. The)' 

 will be seen to be not blue or green (as long as light is 

 allowed to come from the mirror of the microscope up 

 through them) but either colorless or of a pale )'ellowish 



