Fatio on the Forms and Colours of Plumage. 385 



of form ; but it is not so with the ordinary or the mixed 

 feathers. They modify their colours either by augmenting 

 the intensity of their original tint, or by replacing an old 

 colour with a new one. In the first case there is a more com- 

 plete solution of the internal pigments, and in the second an 

 extravasation of the old colouring matter which disappears as 

 an external dust, while a new solution is made of another 

 pigment latent in the barbs. In this case it is rather difficult 

 to obtain a complete solution by operating from without ; and 

 I think that since those birds which entirely change the colours 

 of their great wing and tail feathers, modify their little feathers 

 much less, it is probable that a particular grease developed 

 in the body, and reaching the feather internally, may be 

 necessary to a solution of the new pigment, which, as we have 

 seen, is already deposited in the interior of the feather/'' 



" The feathers I have called enamelled, comprise all the blue 

 feathers without metallic reflexions, and some of the most 

 brilliant green feathers which are without these reflexions. . . . 

 These enamelled feathers always exhibit in the interior of their 

 barbs at their birth as well as after a complete moult, large 

 polygonal cells with coloured nuclei. Below the colourless 

 epidermis and on the dorsal or upper face of new feathers, the 

 microscope always shows a layer of elongated vertical cells, 

 and their thickness gradually diminishes in advancing towards 

 the lower surface. These cells constitute what I call the 

 enamel. 



" This transparent enamel is differently coloured and variable 

 in thickness. Its tint gives the appearance of green or blue, 

 and its thickness gives a greater or less fixity to the colours 

 which often change from the finest blue to the finest green, 

 and sometimes to a tint of delicate yellow. A very thick 

 layer gives the appearance of an opaque colour. In spring 

 the colouration increases, the barbs develope a little, and many 

 barbules disappear, while in the meantime the dark internal 

 pigment gradually dissolves, passing through transitive tints. 

 The enamel only seems to be slightly soluble in the dissolving 

 material in the axis." 



M. Fatio gives the following summary of these various 

 actions : — 



"1. Of two successive axes the first developes always at the 

 expense of the latter. 



"2. In ordinary feathers, properly so called, the secondary 

 axis predominates over the tertiary, and the chief changes 

 take place in the barbs. 



" 3. In optical feathers, properly so called, the tertiary axis 

 predominates, and the chief modifications are effected in the 

 barbules. 



vol. x. — no. v. c c 



