OCR Text |
Show 1882.] DR. H. GADOW ON THE COLOUR OF FEATHERS. 413 together with the proper substance of the feather, occupies the rest of the barb. Thus we have, if proceeding from the surface to the middle of a blue barb; the following structure (fig. 1) :- 1. A transparent, apparently homogeneous sheath of ceratinine (S S). 2. One layer of prismatic cells; and 3, under this, a brownish pigment (P). The sheath may vary in thickness and in surface-structure from about 0 0014 to 0*0043 m m. In Pitta I calculated its thickness to 0*0016 mm., and the surface appeared to be quite smooth ; whilst in Ccereba each top of a cone corresponded with a slight elevation of the sheath. The breadth, or diameter, was calculated to about 0*006 mm.; it agrees very closely with that of Ccereba and Ara. Fig. 1. Wm$mm &*m Diagrammatic section through part of a barb of a blue feather. Fatio, who examined the structure of blue feathers, also says that under the prismatic layer there are " de grandes cellules polygonales a. noyau colore." But I suppose that this is an optical delusion, and that the large polygones (generally hexagones) which we see while looking vertically down upon the surface of the rami are the lateral outlines of the prismatic columns. Therefore what he figured (op. cit. plate iii. fig. 6) as polygones are simply the foreshortened columns, and the underlying pigment gives them the appearance of cells with a dark nucleus. The thickness of the surface-coating of blue feathers varies considerably in different birds, and even in different feathers of the same bird. Differences between 0*0016 and 0*0043 cannot be put down as mistakes of measurement. Again, we know that the thickness of colour-producing plates varies from about 0*00006 to 00004 mm., giving bluish-white or pale orange light respectively. And if the plates in question are thicker than about 0*0005 mm., they cease to produce colour, and the law of colours of thin plates is |