Plate 1., Fig. 1, is a photograph of the Lesser Covert of a Grouse, enlarged 

 six times direct in the camera without the aid of a microscope. This magnificat 

 tion is sufficient to show the general construction of a feather, which consists of 

 a shaft (Rhachis) bearing a multitude of branches called barbs. Each of these 

 barbs bears on both sides a diminutive set of branches called Barbules, and the 

 whole mass is called the Vane. 



One of the barbs has been specially separated to show the barbules which 

 appear in the form of a shadow on each side of the barb. The barbules on the 

 upper side are called Distal Barbules, those on the lower side are called Proximal 

 Barbules. The distal barbules are armed with hooks (hamuli) which grip the 

 proximal barbules belonging to the adjacent barb and form the resistance which 

 is felt when the vane is pulled apart. If the disturbed vane is smoothed again 

 into its original position the hooks at once resume their grasping function. In fact, 

 the whole principle of the strength of the vane to hold firmly together against the 

 resistance of the air when a bird is flying lies in this locking together of the 

 barbs. 



In Fig. 1 that portion of the vane consisting of barbs closely held together 

 is called Pennaceous, while at the base the soft and downy structure composed of 

 barbs separated one from the other is called Plumulaceous. In some cases a 

 feather is entirely pennaceous and frequently entirely plumulaceous. 



Plate I., Fig. 2. Some feathers are double, as from the umbilicus another 

 feather rises at the back in a more or less developed form. This secondary feather 

 is called the Hypoptilum, and the stem Hyporhachis. This photograph, which is of 

 a Grouse Contour, was taken direct in the camera with a magnification of 2'75. 



The hypoptilum lies close behind, and is covered by the main feather in 

 nature. To make it more visible it was bent away to some distance. In this 

 case the hypoptilum is entirely plumulaceous, but in some cases it is partly 

 pennaceous. 



