350 ADAPTATIONS TO SPACE AND MOTION 



motion, from a much greater distance than that from which we can 

 resolve it if it is still. Ovio has given a simple explanation of this : If we 

 take as our criterion of visual acuity the two-point limen, or angular 

 separation which two points must have if they are to be just resolvable 

 as separate points, then their subjective separateness is due to the fact 

 that their images on the retina fall upon 'circles of innervation', or groups 

 of visual cells (one in each 'group', in the fovea) connected with single 

 optic nerve fibers, which have between them an unstimulated circle of 

 innervation. The actual separation of the points in space thus corres- 

 ponds, in the retinal image, to the diameter of a circle of innervation in 

 that part of the retina. But now if the displacement of a single point in 

 space is to be visible, the image of that point need move on the retina 

 a distance equal only to the radius of a circle of innervation, in order to 

 fall upon a new circle and register the displacement. 



This idea is quite well borne out by the experimental facts. The two- 

 point limen at the human fovea is about 40" of arc. The angular dis- 

 placement-threshold at the fovea is 20" of arc or less, according to dif- 

 ferent observers. Schmid, studying the visual performance of fourteen 

 police dogs, found that the best dogs could recognize moving objects at 

 810-900 meters, while the best record with the object stationary was 

 585 meters — not far from the 2 : 1 ratio which Ovio's explanation roughly 

 predicts. Thus, paradoxically, it seems that we should, after all, discrim- 

 inate changes in a visual pattern better than the static features of the 

 pattern itself. 



When a visual stimulus is presented, there is a 'latent period' before 

 the sensation develops, and the sensation lasts longer than the presen- 

 tation-time or duration of the stimulus. The 'persistence time' is the 

 period within which a stimulus continues to be sensed after it has been 

 removed. In vision, this period is synonymous with the duration of the 

 'immediate positive after-image' of a stimulus. It is commonly stated to 

 be responsible for making motion pictures 'move', though it is directly 

 involved only in the elimination of 'flicker' from them. If an after-image 

 has not commenced to fade before an identical second stimulus evokes 

 its full-strength sensation, the second sensation or impression will merge 

 with the first. As successive flashes of light are thrown on the same ret- 

 inal area, an increase in their frequency leads ultimately to the percep- 

 tion of a steadily-burning light, at the 'critical frequency of fusion'. 



Now, at this critical frequency, the interval between the cessation of 

 one stimulus and the commencement of the next might be called a 



