98 COLOUR 
long tail feathers. The red colours of the spectrum lie nearer 
towards the position A, the blue colours towards C. The colours 
always appear in the same order: no feathers are known, which 
t 
POSITIONS FOR OBSERVING THE COLOUR OF ‘‘ METALLIC” FEATHERS. 
(From the Proceedings of the Zoological Society, 1882. 
when looked at from B towards A, change from the red towards 
the blue end of the spectrum. In case two or more of these spectra 
(of which we imagine the horny coating to be composed) overlap each 
other, only a limited number of colours are able to reach the eye 
of the observer. Thus in the theoretical case figured red only will 
be visible besides black. 
A peculiar case is that of Artamia bicolor; the pure white feathers 
Al 
MK KKK 
pAneee — 
ane 
ee 
As 
DIAGRAMMATIC SECTION THROUGH THE BARB OF A ‘‘ METALLIC” FEATHER. 
(From the Proceedings of the Zoological Society, 1882.) 
of the underparts have no metallic gloss, but nevertheless they seem 
to be prismatic, because in position A the underparts appear bluish- 
white, in B delicately pale blue, and in position C pale grey. 
Deviation from the normal coloration is more or less patho- 
