. The Emu. Birds -- Periodicals; Birds -- Australasia Periodicals. 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 o
. The Emu. Birds -- Periodicals; Birds -- Australasia Periodicals. 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. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Australasian Ornithologists' Union; Royal Australasian Ornithologists' Union. Melbourne : Australasian Ornithologists' Union
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Keywords: ., bookcentury1900, bookdecade1900, booksubjectbirdspe, bookyear1901
