Ornithology. 39 



Nictitating Membrane of Birds. As birds are continu- 

 ally passing among hedges and thickets, their eyes are pro- 

 tected from external injuries, as well as from too much light 

 when flying in opposition to the sun's rays, by a nictitating or 

 winking membrane, which can at pleasure be drawn over 

 the whole eye like a curtain. This covering is neither 

 opaque, nor wholly pellucid, but somewhat transparent. By 

 means of it the eagle is said to gaze at the sun. 



Vision of Birds. Ross in his voyage to Baffin's Bay, 

 proved that a man under favorable circumstances could see 

 over the surface of the ocean to the extent of 150 English 

 miles. It is not probable that any animal exceeds this power 

 of vision, though birds, perhaps excel men and most quadru- 

 peds in sharpness of sight. Schmidt threw at a considerable 

 distance from a thrush [Turdus musicus) a few small beetles 

 of a pale gray color, which the unassisted human eye could 

 not discover, yet the thrush observed them immediately and 

 devoured them. The bottle tit [Parus caudatus) flits with 

 great quickness among the branches of trees, and finds on 

 the very smooth bark its particular food, where there is noth- 

 ing perceptible to the naked eye, though insects can be de- 

 tected there by the microscope. 



In a recent number of the Medico Chirurgical Review, Dr. 

 James Johnson has shrewdly combatted the opinion, that 

 birds of prey could scent, and thinks that they are usually, if 

 not uniformly, guided by vision. 



'It has always appeared to us,' says he 'most extraordinary, 

 indeed unacountable, that birds of prey scent carcasses at 

 such a distance as they are said to do. We were led to 

 scepticism on this subject, some twenty years ago, while ob- 

 serving the concourse of birds of prey from every point of the 

 horizon, to a corpse floating down the river Ganges and that 

 during the north-east monsoon, when the wind blew from one 

 point of the compass for months in succession. It was ex- 

 tremely difficult to imagine that the effluvia from a putrifying 

 body in the water could emanate in direct opposition to the 

 current of air, and infringe on the olfactories of birds many 

 miles distant. Such, however, were the dicta of natural his- 

 tory, and we could only submit to the general opinion. We 

 have no doubt, now that we know the general opinion to be 

 something wrong, that it was by means of the optic rather 

 than the olfactory nerve, that those birds found out their 

 quarry. 



