when reduced into thin Pellicles. la 



yellow shades by a perpendicular reflection, and greenish by 

 an oblique reflection; while, more interiorly, by the same 

 change of obliquity, a space of the most lively green as- 

 sumes the new shade of the violet. Such are the principal 

 mutations of these colours, consisting of two shades only 

 for each spot. 



Upon a feather from the neck of a pigeon the disposi- 

 tion is quite contrary to that of the lateral beards of the pea- 

 cock's tail; i.e. in the same circumstances one of these 

 feathers gives red, while the other gives green, and vice 

 versa. 



It is very difficult to reconcile this alternation of colours, 

 confined to two principal species, with the variety of shades 

 which the coloured rings would seem to give by means of a 

 substance of a density so feeble as that which constitutes 

 feathers. Should we attempt to draw an inference from the 

 more sensible changes which the feather of a pigeon pre- 

 sents in its shades, this would still afford but a deceitful 

 analogy: these changes proceed only from the generally 

 bent state of the feather, since they cease when we lay it 

 against a flat surface. 



But what forms a total disparity is the appearance of the 

 feathers of the wing of a duck. Here the trdnsition takes 

 place from green to blackish ; and again, this green colour 

 is not sensible, except in peculiar positions, where the in- 

 cidence and reflection of the light take place under very un- 

 equal angles, as, for example, when we view the feather 

 under a certain obliquity, having our back turned to the 

 hght. Do we never see any thing similar in the succession 

 of coloured rings ? 



I thought, lastly, of carefully moistening several places of 

 the region of the eye of my peacock's feather. I then saw, 

 not a weakening of the first shades, but new colours emerg- 

 ing with a good deal of strength. I was anxious to know 

 if I could not produce permanent alterations by some sol- 

 vent: I consequently tried to moisten it successively with 

 sahva, with vinegar, weak and strong muriatic acid, with 

 ammonia, ether, alcohol, and with a solution of muriate of 

 lime ; and I ascertained that thesQ agents had no other in- 

 fluence 



