'112 DR. H. GADOW ON THE COLOUR OF FEATHERS. [May 2, 



can actually be done. If we press one of the deep-blue feathers 

 of a Maccaw between two hard planes, so as to squash or smash the 

 stratum of prismatic cones, or if we hammer it carefully, the blue 

 immediately disappears, and the injured part looks grey or brownish 

 according to the underlying pigment. The same is the case with 

 the beautifully blue feathers of Artamia. Green parrot-feathers, when 

 treated in a similar way, become yellow, since this is the colour of 

 their pigment. Thus structural or optical colour may, so to speak, 

 be knocked out of a feather. (Fatio observed that blue disappears 

 after injuring the surface by scratching off some of the enamel.) 

 This explains the dark appearance of the abraded parts of feathers 

 of Parrots and other vividly colonred birds. Again, red, orange, 

 brown, black, and most of the yellow feathers (i. e. such which owe 

 their colour directly to pigment) do not lose or change their colour 

 under any physical treatment. 



The explanation of the blue colour is the most difficult of all in 

 those feathers where the blue is independent of the position of the 

 eye, i. e. in which the blue does not change. In most cases the blue 

 is confined to the rami, which, for instance in Careba and in 

 Artamia, in the blue parts of the feather are devoid of cilia and radii, 

 and are broader and flattened out {cf. Fatio). 



With a magnifj'ing-power of about 640, we first observe that the 

 whole ramus is covered by a transparent, slightly yellowish, or per- 

 haps quite colourless, sheath or coating, the thickness of which is not 

 more than 0*00 14 of a millimetre. The surface of this sheath is 

 uneven and granulated. Immediately under this sheath we find one 

 continuous layer of prismatic polygonal (frequently hexagonal) cells 

 or cones. Most of these cones are broadest at their apices, and be- 

 come smaller towards their bases ; others have nearly parallel walls 

 or may be broadest below. (This layer of cones has been called 

 by Fatio, its discoverer, "email.") The space between their apices 

 seems to be filled up with the same matter as the coating. The 

 colour of the cones is pale yellowish, or, if this is only the reflection 

 of the underlying pigment, they are colourless. The distance 

 between the middle of two neighbouring apices I found equal to 

 0*0050 of a millimetre ; this would also be their breadth at the 

 base. Their height seems to be slightly larger. No actual mea- 

 surement, however, could be obtained, as I did not succeed in 

 getting a clear side view of them. As to the structure of these 

 little cones themselves, it is very difficult to arrive at a satisfactory 

 conclusion, considering the minuteness of the subject. However, in 

 Pitta moluccensis and in Artamia I observed a system of extremely 

 fine lines running parallel with the long axis of the cones, i. e. trans- 

 verse or vertical to the long axis or surface of the ramus. These 

 lines themselves do not seem to be straight, but irregularly waved. 

 The breadth of each bar I calculated to be less than 0"0006 of a 

 millimetre. 



Below this stratum of polygonal prisms or cones lies brownish- 

 yellow pigment, near the middle of the barb ; where the layer of 

 pigment is thicker it looks black-brown. This pigment, of course, 



