"To summarize, hummingbird iridescence is due to interference colors produced by a stack of about three films whose optical thickness is one-half the peak wave length. Each film is a mosaic of platelets of elliptical form. Each platelet is about 2.5 microns long and one micron wide. The platelets are not homogeneous and consist of air bubbles encased in a matrix of refractive index about two. The different hummingbird colors are produced by a combination of effects. The platelet thickness decreases moderately as one passes from red through green to blue, and the air content increases simultaneously. In theory, of course, platelet thickness could remain constant and the color change would arise solely by variation in air content. Conversely air content could be constant and the platelet thickness varied. Nature for reasons best known to herself has elected to vary both factors together.” (Greenewalt et al. 1960:253)
"First, we repeated the examination of barbular surfaces with the optical microscope. We confirmed the presence of the platelet mosaic on the iridescent surfaces of fifty or so hummingbird species. Only the platelets are colored; the interstices are dark. The platelets are minute, about 2.5 microns across the long axis of the ellipse, and one micron across the short axis. Ten thousand of them laid end to end would measure a little over an inch. Their size varies little throughout the hummingbird species which we have examined. The length of 2.5 microns is a good average and the limits of variation would be no more than 30 per cent either way. Along its length the barbule is divided into cells separated by diagonal lines crossing the width of the barbule. At the points where the barbule joins the ramus and where the pennulum develops, the colored platelets disappear and one sees only a few random uncolored or faintly colored ellipses in these areas. The barbule proper then has a surface which is 15-20 microns wide and 100 microns long, divided by diagonal boundary lines into a series of cells which look like parallelograms, each cell made up of a mosaic of 100 or more beautifully colored elliptical platelets” (Greenewalt et al. 1960:250).