1882.] DR. H. GADOW ON THE COLOUR OF FEATHERS. 409 
The broad nuchal collar of orange of N. cornutus is altogether 
wanting, and there is no difference between the colour of this part 
of the back. 
We observe that the brilliant blue of the primaries of NV. cornutus 
is much dimmer in NV. weensis; but this may result from our 
specimens being caged birds, and consequently not in such perfect 
plumage. In size and shape the two species are identical. 
5. On the Colour of Feathers as affected by their Structure. 
By Dr. Hans Gapow. 
[Received May 2, 1882.] 
(Plates XXVII. & XXVIII.) 
The colours which we perceive in the things surrounding us may 
result from various sources, according to which they may be divided 
into two classes. 
The First CLass consists of so-called objective, chemical, or 
absorption colours. Such colours do not change or vary under any 
position of the light or eye ; they receive their colour always from a 
colouring-matter. This may exist in the form of a solution or as 
pigment. Animal objective-colours are mostly due to pigment. 
The seconp crass has been variously described as subjective, 
physical, or structural colours. They are the result of reflected or 
broken light. This may take place in various ways : — 
1. Total reflection of light, e.g. the gloss on a polished surface. 
2. The light may be broken infinitely often and be totally re- 
flected. The result of this is white. 
3. Diffraction by a prism. 
4. Interference of colours. By this theory are explained the 
colours of extremely thin transparent plates, and’ those which are 
produced by a system of narrow ridges, e. g. iridescence of mother-of- 
pearl, the blue-heat stage of steel, and the colour of soap-bubbles. 
As I have not, in this communication, entered into any general 
history of the colours of birds’ feathers, but have confined myself to 
the effects of structure, it appears unnecessary to give any detailed 
critical account of the work of my predecessors, which should only 
make part of an elaborate and complete essay. ‘he titles of the 
more important papers, however, are given in the footnote! 
1 B. Altum, “Ueber die Farben der Vogelfedern im Allgemeinen, und iiber 
das Schillern insbesondere,” Naumannia, 1854, p. 293. 
B. Altum, “Ueber den Bau der Federn als Grund ihre Farbung,” Journ. 
f. Orn. 1854, pp. xix—xxxy. 
A. Bogdanow, “Note sur le pigment des plumes des Oiseaux,” Bullet. de la 
Sce. d. Naturalistes de Moscou, 1856, p. 458. 
V. Fatio, “Des divers modifications dans les formes et la coloration des 
plumes,” Mém. Soc. phys. hist. nat. Genéve, xviii. (1866), p. 249, plates. 
K. W. Krukenberg, ‘ Vergleich. physiolog. Studien,’ ser. 1. pt. v., ser. 2. pt. i. 
[See also a further contribution (ser. 2. pt. ii.) which has only dome into 
my hands as the revise is leaving them.—H. G., July 25¢h.] 
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