4«4 THE BIRDS xvi. 29 



10 in man). The kingfishers are said to possess an amazing arrangement 

 of double foveas, placed at different distances from the lens, so that 

 as the bird dives under water the image is transferred from one fovea 

 to the other without any change in the dioptric apparatus. These details 



Herring Gull 

 (Larus) 



Shearwater 

 r Puffinus) 



Great: Bustard 

 (Otis) 



Coot 

 (Fulica) 



Ostrich 

 (Struthio) 



Cormorant 

 (Phalacrocorax) 



Humming Bird 

 fCalypte) 



Shrike 

 fLanius) 



King Fisher 

 (filcedo) 



Fig. 295. The appearance of the retina of various birds as seen with an 



ophthalmoscope through the pupil. 



cm. central area;/, fovea; p. pecten; t.a. temporal area. (After Wood, from Pumphrey.) 



of the visual system show, like so many other features of bird ana- 

 tomy, how readily the structure conforms to special habits of life. 



The retina of day-birds consists largely but not wholly of cones; 

 these animals are more fully diurnal than is man. The high resolving 

 power and hence high powers of discrimination and of movement- 

 detection depend on the great density of the cones, as many as 

 1 million to each square millimetre in the fovea of a hawk, three times 

 denser than in man. Nocturnal birds, on the other hand, have retinas 

 composed mainly or completely of rods, and the differences between 

 the behaviour of these two types of eye, found in birds as in mammals, 

 have been a powerful support for the duplicity theory of vision. There 

 are usually one or more areae, regions of the retina consisting of tightly 



