426 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1011. 



group of molecules may cause selective reflection. It seems clear 

 that this property can not belong to individual molecules, at any 

 rate in the case of the aniline dyes, for their solutions absorb impar- 

 tially all the colors which are not transmitted, and it is only in the 

 solid state that their peculiarities as regards reflected light become 

 apparent; at the same time there is no change in the light transmitted 

 whether the dye is in solution or a dry film. 



Before entering in detail into the reasons which seem to point to 

 interference rather than selective reflection as the origin of iridescent 

 colors, some general remarks may be made on the character of the 

 structures examined. 



These structures have been either feathers of birds or the scales 

 of insects. There are few orders of birds in which examples of 

 iridescent coloring can not be found, but without doubt the humming 

 birds are the most brilliant, although peacocks, trogons, and many 

 others^ are not very far inferior. In the insect world the finest 

 examples are to be found amongst butterflies and the day-flying 

 moths of the genus Urania. Some beetles also are provided with 

 vividly colored scales. These belong mostly to the weevils (which 

 include the Brazilian diamond beetle). 



Many^other insects among the Diptera, Neuroptera, and Hymenop- 

 tera show brilliant metallic colors on their integuments, but these 

 are not provided with scales, and in many cases the color fades 

 more or less when the specimens become dr}^. These I have not 

 examined. Feathers and scales, however, are remarkable for the 

 permanence of their iridescent coloring, and it is to these only that 

 the present observations apply. 



Some of the peculiarities of the structures as regards change of 

 color with the point of view depend on the shape of the surface 

 on which the color-producing material lies. If the surfaces are 

 flat or nearly flat, reflection takes place as from a looking glass, 

 and the angle through which the specimen can be turned while 

 still showing the characteristic color is small. Often, however, 

 the surfaces are convex bosses or ridges, and then the angle of inci- 

 dence and reflection is that contained between the direction of the 

 incident light and the normal to the tangent plane at the point 

 where reflection takes place, and is therefore to a great extent inde- 

 pendent of the position in which the specimen is held, since there will 

 always, within wide limits, be tangent planes to the convex surfaces 

 which reflect the incident light in the line of sight. In these cases 

 the colors might at first sight be taken as due to pigment, both on 

 account of their comparatively low intensity and from the small 

 change in tint and intensity which is produced by altering the inclina- 

 tion of the general surface to the direction of the illumination. The 



