1882.] DR. H. GADOW ON THE COLOUR OF FEATHERS. 409 



The broad nuchal collar of orange of N. cornuius 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 N. cornutus 

 is much dimmer in N. uvceensis ; 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. Ou tlie Colour of Feathers as affected by their Structure. 

 By Dr. Hans Gadow. 



[Eeceived 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 

 colourine:-matter. This mav exist in the form of a solution or as 

 pigment. Animal objective colours are mostly due to pigment. 



The SECOND CLASS 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. The titles of the 

 more important papers, however, are given in the footnoted 



^ B. Altum, " TJeber die Farbeii der Vogelfedern ini Allgemeiueu, und iiber 

 das Schillem insbesondere," Naumannia, 1854, p. 293. 



B. Altum, " Ueber den Bau der Federn als Grund ibre Fiirbung," Journ. 

 f. Orn. 1854, pp. xix-xxxv. 



A. Bogdanow, " Note sur le pigment des plumes des Oiseaux," Bullet, de la 

 Soc. d. Naturalistes de Moscou, 185ti, p. 4.58. 



V. Fatio, " Des divers modifications dans les formes et la coloration des 

 plumes," Mem. Soc. pbys. hist. nat. Geneve, xviii. (1866), p. 249, plates. 



K. W. Krutenberg, ' Yergleieh. pbysiolog. Studien,' sei-. 1. pt. v., ser. 2. pt. i. 



[See also a farther contribution (ser. 2. pt. ii.) which has only dbme into 

 my hands as the revise is leaving them. — H. Gi.,Jitly •2bth.'\ 



28* 



