486 THE BIRDS xvi. 29 



and increase acuity. Pumphrey points out that any such advantage 

 would be counteracted by the aberration introduced and he makes the 

 suggestion that this disturbance of the picture by the 'convexiclivate' 



x 



iT 



^ 



Fig. 297. Effect of refraction produced by the curvature of the fovea 

 of the golden eagle. An image of the form shown with dotted lines is 

 distorted by the fovea to the form shown in solid lines. The circle 

 represents a radius of io/j. at the centre of the fovea. (From Pumphrey.) 



Fig. 298. Distortion by the fovea. The lines represent the successive images at 

 equal time-intervals of the boundary of a regular object when the object moves 

 steadily across the visual field. If this picture is viewed at 7 m the area of irregu- 

 larity subtends an angle about equal to the angle subtended by the central part of 

 the hawk fovea. It will be found that the irregularity is very evident to the human 

 eye at this distance though the lines are resolvable with difficulty. 

 (From Pumphrey.) 



fovea is itself an advantage, improving the power of fixation and sensi- 

 tivity to movement, at the sacrifice of acuity. Such an arrangement 

 would serve to emphasize angular displacements, transforming a 

 radially symmetrical image into an asymmetrical one, except when 

 there is coincidence between the axes of symmetry of the fovea and of 



