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 000 14 to 0-0043 mm. 



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. 



Diagrammatic section through jyart 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 con- 

 siderably 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 O'OOO.") mm., 

 they cease to produce colour, and the law of colours of thin plates is 



