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 (in 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 

 Euploea, Morpho, Calligo, Argynnis (in which silver markings are 

 common), Vanessa, Callicore, Lycsena, Thecla, Papilio, Ornithoptera, 

 some of the Hesperidse and moths of the genus Urania. The only 

 beetles examined were Entimus imperialis 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 well 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 (if 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 



