*73 



ieen L'radually to iill the interior. .UK! simultaneously all ' color 



vam'^ho (except sometimes a i.iini greenish -urfa< e color). It apj 



then, that the color in tin due to line BtriatioiU 00 tin- interior 



ale. i Michrlson.) 



'I'hc interference colors nt" huttertly BCftlea may he due, not only to 

 surface markings, hut also to tin- lamination of the scale and to the 

 overlapping of two or more >cales. In heetle- the hrilliant blue 

 freeHS, and iridescence in general, are sometimes produced hy minute 

 lino or pits that diffract the light. Acionlini: to Tower, "The pits 

 aloiu 1 , howi-vcr, are powcrles> to produce any color; it is only when they 

 are combined with a highly reflecting and refractive surface lamella and 

 a pigmented layer below that the iridescent color appears. The action 

 of light is in this case the same as in the plain metallic coloring, excepting 

 that each pit acts as a revolving prism to disperse different wave-lengths 

 of light, in different directions, and the combined result is iridescence. 

 Tin- existence of minute pits over the body surface is of common occur- 

 rence, but it is only when they are combined as above that iridescent 

 colors occur." 



The production of color by ''metallic" reflection deserves more 

 attention than it has received from naturalists. The metallic colors 

 of birds and insects have been studied precisely by Professor Michelson, 

 who has proved that they are due to the same causes in both animals 

 and metals. The metals, on account of their extraordinary opacity, 

 throw back practically all the light that strikes them, thus giving the 

 characteristic brilliant reflections; the distance to which light can pen- 

 etrate in most metals being only a small fraction of a light wave, so 

 that a wave-motion such as constitutes light, strictly speaking, can 

 not be propagated at all. As this opacity may be different for different 

 colors, some would be transmitted more freely than others, so that the 

 resulting transmitted light would be colored; and the reflected light 

 would be approximately complementary to the transmitted color, 

 i Michelson.) Thus the reflected light from the metal gold is yellow, 

 the transmitted light being, blue. In certain pigeons, peacocks, hum- 

 ming birds, as well as a number of butterflies, beetles and other insects, 

 the brilliant metallic colors are due to an extremely thin surface film 

 which has optical qualities like those of metals. This film in the case 

 of the coppery wing cover of a beetle was calculated by Micheson to be 

 less than a ten-thousandth of a millimeter in thickness. 



In animals, as in metals, these colors are brilliant because a large 

 ntage of the incident light is reflected. The color of the reflected 



