specific modifications of Japan Carahi. 513 



Eegarding the metallic colours of birds, the same 

 mechanical action of the sun's rays seems to produce 

 them. The downy feathers of the brightest are almost 

 colourless, for the light-rays touch the exposed ends of 

 the feathers only. If a swan were dipped into a tub of 

 scarlet dye it would produce a similar effect, and leave 

 the down white. In the humming-birds and swifts, 

 which are continually flying during the daylight, the 

 colour of the under surface of the wings and the sides of 

 the body they cover when at rest corresponds to that of 

 the upper, and the same would occur in dyeing a swan if 

 during emersion its pinions were opened. As the sun- 

 birds fly both sides of their wing-feathers are exposed to 

 the same rays or waves, and the same impressions or 

 ribs are stamped on either side of them, for during the 

 day they are ever on the wing. Birds which perch much 

 have colourless under sides to their wings. Let the 

 humming-bird be thought of in the hovering position of 

 sipping honey, or coquetting on a twig in the midday 

 sun : just as the gorgeous butterflies soar and catch the 

 vertical or direct rays, such, in fact, as appear necessary 

 to produce the right ribbed structure for metallic bril- 

 liancy, so these birds are incidentally, by their position, 

 so placed as to be struck by the most direct rays in such 

 a manner as to give brilliancy and metallic colour to the 

 head, breast, &c. The tail is lowered in some species, 

 so that the breast and scalp catch the full power of the 

 sun, and the wings are struck obliquely like those of the 

 brown flittering, or more retiring, species of Lepidoptera. 

 The sun-rays, of course, are not affected by the move- 

 ment of the wings, but by their varied positions the 

 wings are impressed differently. To appreciate fully the 

 extreme delicacy of the ribbed structure, produced as I 

 think by the sun, is almost impossible ; but to see its 

 effect we must face the birds with our heads between 

 them and the light, and then the greatest brilliancy 

 appears, for at any other angle the sculpture is different, 

 less fine, and the brightest colours are lost. It is the 

 same difference as if we looked on the sand from the 

 Folkestone beach, or saw the ripples while wading in the 

 sea. A few of these birds are metallic in a reverse 

 attitude, and it is probable their habits of flight or posi- 

 tion in rest are only approximate, for in our classification 

 they stand in different genera. Some have long tail- 

 feathers which float in the air according to the move- 



