Jeffries on the Colors of Peat hers. I 33 



Blue colors seem to be accidental, that is. the result of other 

 causes than pigments. Not only have all etibrts to extract the 

 pig-ments failed, but blue feathers appear gray when examined bv 

 transmitted light. Again, no blue can be found in transverse sec- 

 tions of blue feather parts. This method of studying the colors 

 of feathers is worthy of more extended use than it has yet had. 

 B}- this means all physical effects of the outer coat are avoided, 

 and tlie exact position of the pigments can be seen. Sections 

 are cjuickly prepared by fastening the feather on to a piece of 

 pith with collodion, and mounting sections pith and all. The 

 pith keeps the sections on end, a result otherwise difficult to 

 obtain. 



Grav-blues, such as thcjse seen in DeJidrceca cceriilesceiis, are 

 due to opalescence. The feather is full of fine granules of black 

 or darkish pigment, which in a manner already described produces 

 a blue color. 



Brilliant blues, as those shown b\- Sialia stalls, Cyanosplza 

 cva//ea. Ca^reba l/iclda, and the like, do not seem to be suscep- 

 tible of a like explanation. The color is too intense and pure to 

 be produced in such a small space by opalescence. So most 

 authors have simply ascribed it to some other form of interfer- 

 ence, as a thin outer plate, which would seem on examination to 

 be the true cause. Figure 2, drawn from a section of a Bluebird's 

 barb enlarged about one thousand diameters, will give an idea of 

 the structure found in such cases. The central cells are full of 

 some dark pigment, probably zoomelanin, while the surface is 

 bounded by a transparent layer of horn varying from ^Tji^^Tr to 

 -^-^^^ of an inch in thickness. Thus we have a contri\ance not 

 ill ada'' • ^o the production of interference colors, the black 

 pigment ai)sorl)ing all rays which are not reflected by the horn 

 coat on the outside. Yet there arc decided difficulties in this 

 view. Thin as it is, the outer horn coat is thick comparefl to the 

 length of light waves, and again the blue color is constant. How- 

 ever, in spite of these objections, the color must be ascribed to 

 the action of the outer coat of cells. The structure of other 

 bright blue feathers is much the same, though difierences in 

 minutiae exist. Thus the outer layer of cells, the external walls 

 of which form the outer coat of the barb, are devoid of pigment 

 in the Blue Jay. (Fig. 3.) 



