382 Fatio on the Forms and Colours of Plumage. 



produce on any feather, but remained intense and ~ brilliant 

 after many washings with alcohol and complete drying 1 ." The 

 same experiment succeeded with the feathers of a finch 

 (Fringilla coelebs) and similar results were obtained in a few 

 minutes by the use of olive oil. 



With reference to the large feathers, M. Fatio remarks 

 that birds in preening their feathers pass their beaks all along 

 each wing and. tail feather, regularly taking a supply of grease 

 from the oil-gland. He adds, " if a change of colour does not 

 begin regularly at the extremity of a wing feather, as is usual 

 with the smaller feathers, it is because it is much more exposed 

 to the action of the air than the latter. And if sometimes 

 the colouration does not proceed regularly in a small feather 

 from the periphery towards the centre, it is for the accidental 

 reason that some parts have been more exposed to moisture 

 and light, or more readily penetrated by the grease." He 

 further observes that the oil-gland becomes larger at the 

 approach of the love season, and that fat birds usually exhibit 

 the most intense colours. 



" The external grease is absorbed by the barbs and barbules, 

 and penetrating by endosmose or capillarity the more or less 

 porous tissues fills the empty spaces, and finding its passage 

 facilitated by dilatations occasioned by moisture, it gradually 

 dissolves the fatty pigments they contain/'' 



' ' Thus under the influence of moisture, alternately absorbed 

 and evaporated, as a preparatory developing agent, afterwards 

 of the grease of the body as a solvent, then of temperature 

 and light facilitating chemical action, the feather colours itself, 

 and changes and augments its tints." 



Referring to the labours of other observers, M. Fatio 

 employs the divisions of Bogdanow, but adds to his " ordinary 

 feathers" others which he calls mixed, and to his " optical 

 feathers" others which he calls enamelled* (emaillees) . " Or- 

 dinary feathers," although often brilliant, do not exhibit 

 metallic reflexions, and they show by their transparency an 

 internal pigment resembling the colour they exhibit by incident 

 light. The " optical feathers" exhibit reflexions, and show by 

 their transparency an internal pigment of a different shade to 

 the colour they assume in incident light. The linnet furnishes 

 " ordinary," and the starling " optical feathers." "From their 

 first appearance out of their sheaths, I see," says M. Fatio, 

 " essential differences between the ordinary and the optical fea- 

 thers which grow after the autumn moult on the breast of these 

 two birds. I perceive, it is true, on all those parts which are 

 about to fall, that they are grey or brownish in the linnet, or 

 white or yellowish in the starling, but I notice in the former, 

 barbules slender and slightly coloured, while in the latter the 



