ON COLOURS IN NATURE. 95 
the comparatively dull surface of the under side reflect a 
bright mauve on the mother-of-pearl expanse of the upper. 
None of this tribe can be effectively represented by a coloured 
illustration, inasmuch as the opaline tints of some cannot be 
depicted at all, while the brilliant blue of others is only 
represented by a dull indigo hue, and can be seen flashing 
like a speculum or mirror for an immense distance as the 
insect wings its rapid flight in its own land through the 
tropical sunlight. With regard to the humming-bird, when 
the beautiful feathers on its breast are examined under the 
microscope, it has been said that it is astonishing to see none 
of the shades the mystery of which one could penetrate. 
They are simply made of a dark-brown opaque substance, not 
unlike those of the black duck. There is, however, a remark- 
able arrangement,—the back of the feathers, instead of being 
a fringed stem, offers a series of small squares of horny 
substance placed point to point. These plates, of infinitesimal 
size, are extremely thin, brown, and, to all appearance, exactly 
alike, whatever may be the reflection they give. They have 
been described as so many little mirrors, but that comparison 
is not correct, for then they would only give back light without 
colouring it. Neither do they act by decomposing the rays 
which pass through them, for then they would not lose their 
iris tints under the microscope. It is to metals alone that the 
metallic plumage of the humming-bird can be compared ; the 
effects of the plates in a feather are like tempered steel or 
crystallised bismuth. Certain specimens emit colours very 
variable under different angles, the same scarlet feathers 
seeming, when turned to ninety degrees, a beautiful emerald 
green. ‘The same process which nature has followed in the 
humming-bird is also found in the wing of the butterfly. It 
is covered with microscopic scales, which play the part of the 
feathers, arranged like the tiles of a house, and taking the 
most elegant forms. They also lose their colour under 
magnifying power, and the quality of reflection shows that 
the phenomena are the same as in feathers. There is, however, 
a difference in the extent of the chromatic scale. Whilst the 
humming-bird partakes in its colours of the whole of the 
spectrum, from the violet to the red, passing through green, 
those of the butterfly prefer the more refrangible ones from 
green to violet, passing through blue. 
As regards iridescent hues, a brief statement will suffice. 
On the upper surface of the wings of certain Swiss species of 
the genus Hrebia, that are of a dark reddish brown, as a rule, 
I have noticed the play of a greenish and also a purplish tint 
