COLOUR 97 



and Megaloprepia, the surface of the rami and radii is smooth 

 and quite transparent, while between it and the pigment exists a 

 ]ayer of small polygonal bodies, similar to those of blue feathers. 



Blue has not yet been discovered as a pigment. Blue feathers 

 contain only orange or brownish pigment ; the blue appears only 

 on the shafts of the rami and lai-ger radii. The structure of blue 

 feathers seems to be always the same : (1) a transparent, colour- 

 less layer of ceratine, from 0"004 to 0"007 mm. in thickness; (2) 

 a layer of polygonal, more or less pyramidal, and often hexagonal 

 columnar cells, each of which is colourless itself, and its walls are 

 highly refractory and not unfrequently striated and ridged ; ^ (3) 

 the horny narrow cells of the inside of the radius, with brown, 

 black, or orange pigment corpuscles. 



The blue naked parts of the skin of Cassowaries contain yellow 

 or black pigment covered by peculiai"ly modified epidermal layers. 



III. Subjective structural, prismatic, or metallic colours. — These 

 colours change according to the position of the light and the eye 

 of the observer, and they always change in the order of those in the 

 rainbow. They are restricted, as a rule, to the radii without cilia, 

 and moreover to those parts of the feathers which are not covered by 

 others. The metallic portions of the radii are composed of one row 

 of compartments, which often partly overlap each other like curved 

 tiles. In the inside black or blackish-brown pigment is collected ; 

 and each compartment is covered with a transparent colourless layer 

 of extreme thinness, e.g. O'OOOS mm. in Sturnus. The surface of 

 this coat is either smooth and polished as in Nectarinia, or exhibits 

 very fine longitudinal wavy ridges when the feather is violet, or 

 numerous small dot-like irregularities as in Galbula. The coating 

 seems to act like a number of prisms, as indicated in the first 

 figure. All metallic feathers appear black when their surface is 

 parallel to the rays of the light in the same level with the eye and 

 the light. To the eye of the observer at A, in the lower part of 

 the first figure, the metallic collar of Ptilorhis magnifica will 

 appear absolutely black ; the eye at B will see it bright coppery 

 red, and at C rich green ; the metallic feathers of the sides of the 

 breast in the same bird will change from black to green at B, and 

 to blue at C. The beautiful Pharomacrus mocinno changes from 

 greenish bronze through golden green, green, and indigo to violet. 

 Oreotrochilus chimborazo in position B exhibits the whole solar 

 spectrum, namely, violet and red on the head, folloAved by orange 

 and green on the back, blue, violet, and lastly purple on the 



■" In Pitta moluecensis I calculated the following measurements : width of 

 one polygon 0"001 mm., height of same 0'015 mm., thickness of its transparent 

 coating about 0'0012 ; distance between two of the longitudinal ridges on the 

 surface of the polygon 0-0005, thickness of the transparent outer layer of the 

 radius about 0'005 mm. 



