OX THE COLOURS OF THE FEATHERS OF BIKDS. 301 



feathers of the jay of Carolina, I have made the same observation on 

 the green feathers of parrots, but in these the stalks of the beards 

 (fig. 5) are separated, and show the fringes with which they are 

 furnished ; sometimes the latter are coloured, but the colour is so dull 

 that, instead of adding to the splendour of the quill, it serves only to 

 qualify it. 



Thus, the brilliancy of the shining feathers is owing to the hardness 

 and polish of the stalks of their beards, and this lustre is greater, in 

 proportion to the shortness of the fringes which accompany them. 

 That of the guit-guit is much brighter than that of the parrots, be- 

 cause the beards on the feathers of this bird are absolutely bare and 

 similar to prickles, while the beards of the parrot's feathers are fur- 

 nished with fringes which are rather long, and often of a dark colour. 



Those feathers of which the colour varies, shine not only with a 

 high polish, but they have also the property of changing their shades 

 according to the angle described by the ray of light which falls upon 

 them. The green violet-breasted cotinga (Ampeli& Cazana) appears 

 of a sea green when the eye being situated between the bird and the 

 light, the luminous ray describes an acute angle ; but it becomes blue 

 as the angle approaches the obtuse. However the beards of the feathers 

 of this cotinga have no appearance which can be regarded as the cause 

 of this change ; they are like all shining feathers, barbed at the base, 

 and smooth and cylindrical along the remainder. 



It cannot be supposed that the surface of these beards is charged 

 with roughnessj or projecting particles, of which one side is blue and 

 the other green ; if it were thus, these two shades might be perceived 

 on turning the bird round without change of place ; but, on the con- 

 trary, in both positions the feathers are constantly blue or green. 



According to Newton's theory, this change of colour is produced by 

 the fringes being scarcely more dense than the surrounding air; the 

 rays, in passing through this medium to the thin plates or laminae, 

 which we suppose are situated on the side face of the body, diverge 

 but little ; and as, according to this theory, the colour of a body 

 depends on the degree of tenuity in these thin plates, it follows that 

 if the rays a, b, c, (fig. 6) fall perpendicularly on the plate d, e, the 

 space h, c, which it traverses in this plate, being much less considerable 

 than that traversed by the oblique rnyf, b, g, the eye moving from the 

 point a, will experience different sensations as it approaches the point 

 f. Thus, according to Newton, the changeable colours of feathers are 



VOL. i. NO. vn. (JULY, 1833.) Y 



