32 SENSE OF SIGHT. 



the Paradise-birds, and the Humming-birds, re- 

 semble each other in the uniformity of their 

 formation ; all are composed of cylindrical bar- 

 bules, bordered with other analogous regular 

 barbules, which in their turn support other small 

 ones, and all of them are hollowed in the centre 

 with a deep furrow, so that, when the light, as 

 Audebert first remarked, glides in a vertical 

 direction over the scaly feathers, the result is, 

 that all the luminous rays are absorbed in tra- 

 versing them, and the perception of black is 

 produced. But it is no longer the same when 

 the light is reflected from these feathers, each of 

 which performs the office of a reflector ; then it 

 is that the aspect of the emerald, the ruby, &c. 

 varying with the utmost diversity under the 

 incidences of the rays which strike them, is given 

 out by the molecular arrangement of the bar- 

 bules. It is thus that the gorget of many spe- 

 cies takes all the hues of green, and then the 

 brightest and most uniformly golden tints down 

 to intense velvet black ; or, on the contrary, 

 that of ruby, which darts forth pencils of light, 

 or passes from reddish orange to a crimsoned 

 red black. 



It is thus, we think, that the everchanging 

 hues of the gorgets of the Humming-birds from 

 black to emerald, or ruby, or crimson, or flame- 

 colour, are to be explained. 



With respect to the senses of the Humming- 

 birds, that of sight must be peculiarly strong ; 



