J 38 ON CHANGEABLE COLOURS. 



flection and transmission, the opacity of which I have spoken 



not admitting it. 



Eye of the fea- Near the eye of the feather an external ring exhibits yel- 



♦h^'"* lowish tints by perpendicular reflection, and greenish by 



oblique reflection ; while more interiorly, with the same 



change of obliqueness, a space of the most vivid green takes 



the new tint of violet. These are the principal mutations 



of the colours, consisting oaly in two tints for each place. 



Figeon*s neck In a feather from the neck of a pigeon, the disposition is 



»h reverse of ^^^ reverse of that of the lateral filaments of the peacock's 



the peacock s , ^ ... 



lail. tail feathers : that is to say, under similar circumstances, 



one of these feathers appears red, the other green, and vice 



versa, 



Tbe variation This alternation of colours, confined to two principal kinds, 



confined to two • difficult to reconcile with that variety of tints, which 



•olours. - . ..." 



coloured rings apparently slrould exhibit m a substance of 



so little density as feathers. And if it were attempted to be 

 supported by the more sensible changeability of tints in the 

 pigeon's feather, this would arise from a deception ; for this 

 changeability is owing to the naturally curved stale of the 

 feather, since it ceases when the feather is straightened 

 against a flat surface. 

 Feather of a But the feather of a duck's wing exhibits an appearance 

 buck's wing, totally dissimilar. Here the transition is from green to 

 blackish; and this green is not perceptible except in parti- 

 cular positions, in which the incidence and reflection of light 

 take place under very unequal angles, as for instance, when 

 the feather is seen with a certain degree of obliquity, the 

 spectator's back being turned to tlie light. Do we meet 

 with any thing like this in the succession of coloured 

 rings ? 

 Eyeofthe-pea- Lastly J bethought myself of wetting with caution difler* 

 cock's feather ^^^ parts of the eye of mv peacock's feather. I then saw, 

 weftedjshowed ^ , • r. i • ' , - i , 



new colours not a weakenmg oi the lormer tmts, but new colours 



brought out with great vividness. Desirous of knowing 

 •rliich were the whether I could not produce permanent alterations by some 

 jamewithvari- solvent, I wetted it successively with saliva, vinegar, muri- 

 ' atic acid both dilute and concentrated, ammonia, ether, al- 

 cohol, and deliquesced muriate of lime: and I found, that 

 they produced no eflect except as fluids, and all nearly 



equal. 



