14 Upon the variegated Colours of Bodies 



tiine in all their brightness. But when we come to pass 

 the finger slightly over this pellicle, nothing but a green 

 powder is obtained ; the red, ycHow, blue, and purple co- 

 lours, which were so brilliant, disappear in an instant. 



My second phaenomenon is relative to the changeable 

 colours of the pluinage of the peacock and some other birds, 

 such as the cock, the pigeon, the duck, and the turkey. 

 Here, after the most attentive comparative examination and 

 the most circumspect reflections, I ended by renouncing the 

 idea that these kinds of colours could be referred to rings.. 

 ISIy conviction is founded on the following grounds : 



I first considered that these colours were not the neces- 

 sary result of a certain tenuity of the parts ; for, on the 

 one hand, several animals certainly present in their hair, 

 their feathers, or in the down which covers them, exam- 

 ples of tenuities varied to the most imperceptible decree, 

 and yel without there beina; any production of colours, la 

 not the white peacock itself a striking example of this ? 



On the other hand, have not several birds and insects co- 

 lours immoveable in their position, and in their reflections, 

 when viewed under every inclination whatever ? The wings 

 of some butterflies have very fixed colours, although de- 

 pending upon a down so very minute that it is hardly per- 

 ceptible. It is also proper to remark, that all these colours 

 indicate opacity, like those of the peacock's feathers ; with 

 this difference, that in those of the wings of flies we perceive 

 the shades relative to the coloured rings -, but these mem- 

 branes have a se:isible transparency, like scales of mica or 

 blown glass. 



I afterwards observed the changes of colours in several 

 feathers of the birds mentioned. In those of a peacock's 

 tail, we see upon the lateral beards of the stalk, when we 

 change their jwsition, the red changing very suddenly to the 

 green. The red takes place in consequence of the almost 

 perpendicular reflection of the light ; the green, in conse- 

 quence of a very oblique reflection ; and there is no alter- 

 native of refit :tion and of transmission ; the opacity of which 

 1 have spoken above does not admit of it. 



Near the eye of the feather an exterior corona exhibits 



yellow 



