432 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1911. 
brilliancy by holding a piece of glass over the smoke of an arc formed 
between metal electrodes, iron, nickel, silver, and copper being used. 
In the most successful trials the rings were brighter than the colors 
of soap films, and, as might be expected, the intensity of the color 
increased with the angle of incidence, but the rings did not show 
with normal reflection nor until the incidence exceeded 30°. 
In the spectroscopic examination of the color, it was found conven- 
ient to focus the much enlarged image of the surface on the slit of 
the spectroscope. By this means, and using the sun or an are lamp, 
there was sufficient light to show the e bopa from a small part 
of a single scale. 
When cutting thin sections of chitin or feathers, it is important 
that the embedding material should be of the same consistence and 
hardness as the object to be cut. For this purpose shellac gave 
the best results. The hardness could be regulated by the time 
allowed for drying. 
The compression tests were carried out by placing the specimens 
on a slide under a convex lens of about a foot radius. The objects 
compressed were generally thin enough to allow of the Newton 
rings of the higher orders being recognized between the lens and 
slide before any compression occurred, and by centering the object 
in these the pressure could be applied in the right place. 
Since writing the above I have examined the colors of some of 
the iridescent Diptera (chiefly of the genus Lucilia), using the pres- 
sure apparatus. It was found that with them, as with the scales 
and feathers, the color disappeared under compression, and it seems 
probable, therefore, that interference of one kind or another is the 
true cause of natural iridescent color in all cases. It may be remarked 
that the intensity and composition of the light refiected from the 
integument of the flies is such as would be accounted for by the 
interference of a single film or pair of surfaces. 
DESCRIPTION OF PLATES 
Fig. 1—Scale of Entimus imperialis. X 490. 
2.—The same partly permeated with celluloid solution. X 490. 
3.—The same. XX 1750. 
4.—Three scales of same completely permeated. 490. 
5.—Cross section of scale. X< 1750. 
6.—Iridescent scale of Ornithoptera Poseidon. > 1170. 
7.—Black scale of same. X 1170. ’ 
