Vol. XV. 

 1916 



Lewis, Some Cuiisideyatioiis on Sight in Birds. 



221 



which must maintain a constant look-out for the approach of 

 danger, and lor that reason it is found mainly in those birds of 

 poor defence, whose safety lies in speedy detection and evasion of 

 their enemies. In these birds there is the range of two extensive 

 \'isual fields, each being equally recorded ancl scrutinized. The 

 moment an object of interest is detected the bird does not direct 

 l)oth eyes towards it, but there is a concentration of one eye, the 

 \'ision of the other being suppressed at will. In some diseases 

 of man. where the axis of one eye has departed from the parallel 

 of the other, each eye sees a field which does not correspond with 

 the other, yet diplopia or double vision is not present, as the one 



Plain Wanderer, 



FROM A PHOTO. BY D. LE SOUEF. 



or the other field of vision is suppressed according to the automatic 

 concentration in one or the other eye. Note a group of Pheasants 

 or Pigeons watching the same object ; one eye only will be directed 

 towards the position. Watch a Fowl or a Pigeon gazing upward 

 at a Hawk ; one eye will be skyward, the other toward the ground 

 In such cases the vision of the downward eye is being suppressed. 

 If suppression were not possible in birds, a position similar to 

 diplopia would be present. An idea of this condition can be gained 

 by pressing one's eye, thus shifting the visual axis of one eye, 

 when a double image is obtained. In the human it is possible 

 to suppress the vision by exercise and education, otherwise the eye 



