IRIDESCENT COLORS OF BIRDS AND INSECTS—MALLOCK. 431 
shows the same scale partly penetrated by a solution of celluloid 
in amyl acetate; figure 3, ditto Gn which the penetration is not so 
complete) more highly magnified ; figure 4, three scales completely pene- 
trated and quite colorless. Figures 5a and 5b are cross sections of the 
scale (thickness of section about one twenty-thousandth of an inch). 
Feathers are impermeable to most fluids, but when acted on by 
acid (nitric, acetic, or hydrochloric) they change color toward the 
red; after washing and drying, however, they regain their original 
tint. 
The subjects from which the above notes have been made include 
among birds various species of hummingbirds, peacocks and pheas- 
ants, sunbirds, trogons, and others; among Lepidoptera, the genera 
Euplea, Morpho, Calligo, Argynnis (in which silver markings are 
common), Vanessa, Callicore, Lycena, Thecla, Papilio, Ornithoptera, 
some of the Hesperide and moths of the genus Urania. The only 
beetles examined were Entimus vmperialis and two species of Cyphus. 
To the physicist who is also a naturalist the great variety in the 
character of the surfaces on which these metallic colors are devel- 
oped, as weil as the beauty and brilliancy of the colors themselves, 
offers matter of exceptional interest, but it would occupy too much 
space to enter here into a detailed description of even the typical 
forms. 
A rather curious fact, however, may be mentioned with regard 
. to-the scales of Lepidoptera. Nearly all such scales when black or 
colored by pigment have the free end deeply scalloped and presenting 
what may be called an ornamental outline, but the scales which show 
metallic reflections are invariably (as far as my observation goes) 
merely rounded off or have very slight indentations. Figures 6 and 
7, plate 3 (which are respectively colored and black scales from Orni- 
thoptera Poseidon), illustrate the difference. 
Although all the colors referred to in these notes are probably the 
result of interference, the ways in which the interference occurs may 
be very various. Feathers by their behavior suggest an action 
analogous to that of a Lippmann film, but it is difficult to imagine 
matter optically dense enough to behave as the silver particles in the 
film being produced in an organic structure. In most of the scales 
it seems that the interfering rays are reflected from the surfaces of 
very thin flat cells, but it is possible that in some cases the effect 
may be due to reflection from a single dimpled surface. The colored 
central images sometimes given by diffraction gratings are exam- 
ples of this sort of interference, but in order that the colors so formed 
should be as brilliant as possible the depressions or dimples should 
be closely but irregularly distributed over the surface (Gf regu- 
lar much of the light goes in lateral spectra), but of uniform depth 
and section. I have succeeded in making colored rings of some 
