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Show 4 1 2 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 coloured 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 Ccereba 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 magnifying-power of about 640, we first observe that the whole ramus is covered by a transparent, slightly yellowish, or perhaps quite colourless, sheath or coating, the thickness of which is not more than 0*0014 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 become 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 tbe 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 measurement, 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. transverse 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, |