KOTES OX BUTTEEFLIES. 
53 
collected and transmitted to the system for use. Some of 
these are figured at a, Plate LXIX. 
The butterfly tribe is one of the order Lepidoptera , or scale- 
winged insects, so called from the multitude of little scales 
with which the wings are covered. 
These are of various forms, colours, and textures, sufficiently 
distinct to mark the species, with a wonderful variety on the 
different parts of the wing. The shapes are clear and compact 
on all the body of the wing; while at the edges there are 
longer shapes, like plumes, or the wing feathers of birds. They 
are attached, each by a separate stalk, to the membrane of the 
wing. Some of these diversified forms are figured at K. They 
come off like dust when slightly rubbed. The lovely colours 
of these scales seem to depend, not always on an actual sheet 
of local colour, but to be due sometimes to the dispersion of 
light by means of fine lines or striae drawn on each scale. The 
relation of the size of lines, closely ruled, to the length of a 
wave of light, is, by the undulatory theory, easily shown to 
account for the dispersion of colours by a ruled surface, in itself 
colourless. Buttons have been made, as curiosities, thus arti- 
ficially ruled with very fine lines, giving a beautiful play of 
changing colour. The colours of mother-of-pearl, and other 
substances, seem due also to the dispersion of light on a 
white ground. Now, in the case of the purple Emperor a 
changing play of colour is seen, between brown and purple, as 
it catches the light at different angles. This appearance can 
be calculated as due to the relative breadths of the lines 
covering the scales (which vary from 10,000 to 30,000 in the 
inch), when compared with the difference between the lengths 
of waves of light. 
These lines on the scales run always from end to end, 
never across. The reason for this seems to be that by this 
arrangement the drops of rain, if falling on the wing, are rolled 
off easily. The wing is nearly waterproof ; single drops running 
off very cleanly, leaving no trace, if the scales are uninjured. 
The arrangement in position on the membrane of the 
wing is a curious point about the scales, and explains the 
wonderful expansion of the wing, during the first half-hour 
after the butterfly comes out of its chrysalis. When first 
emerged the comparative size of the wing to the body is seen 
at fig. G. In about half-an-hour it will be seen to have 
expanded to about the size figured at h. 
To find the rationale of this development, we must take a 
chrysalis when nearly full grown, and gently lift off the skin 
covering the future wing, whose outline is easily traced on the 
side of the chrysalis, as at e. Applying a microscope, we can 
see the scales very closely packed together, slipped under each 
