352 ADAPTATIONS TO SPACE AND MOTION 



barely visible as a moving object. At 43, it became a faint line whose 

 direction could not be recognized; and at 64 miles per hour it was wholly 

 invisible. 



It is sometimes given, as a characteristic of true movement perception, 

 that we do not see the object in all of its intermediate positions. That 

 really depends upon the nature of the object. The body of an automo- 

 bile may be seen clearly at all points in a movement at a given speed. 

 But the tops of the wheels are travelling faster than the car itself, and 

 so the wheels may blur. Before the invention of the motion-picture 

 camera, a famous photographer, Eadweard Muybridge, made photo- 

 graphic studies of the gait of running horses at the behest of a group 

 of sportsmen who wanted to settle an argument as to whether a trotting 

 horse ever has all four feet off the ground at once. Muybridge used as 

 many as forty automatic cameras spaced along the track. When some 

 of his pictures were handed to horsemen, they refused to believe that a 

 horse's legs ever get into some of the positions shown in the photos. 

 But they were forgetting that while the horse's body cannot travel too 

 fast to be seen clearly, its legs in their forward movements travel so 

 much faster than the horse that they blur in human vision, and no one 

 can honestly say that he sees them in all their positions. While the eye 

 is following the horse's body by a pursuit movement, it cannot very well 

 follow, at the same time, the movements of the horse's legs. 



Adaptation, and Center versus Periphery — The higher the visual 

 acuity, the lower the angular displacement threshold — hence, the better 

 a moving object can be seen and the smaller a movement can be detected. 

 The higher the critical frequency for fusion, the shorter the persistence 

 time (and, sometimes, the latent period) — hence, the less the blur of a 

 moving object and the faster it can move and still be seen well as to its 

 nature, direction, and velocity. 



Visual acuity and critical frequency, being fairly easy to determine 

 in man and animals, are thus our best criteria of the comparative objec- 

 tive capacities of vertebrates for movement-perception. But both of these 

 values are very different for the cone-mechanism and rod-mechanism of 

 the retina. In the average retina (one which is duplex and has an area 

 centralis) both values are profoundly influenced by the conditions of 

 light- and dark-adaptation and by the differing concentration of rods 

 and cones in the center of the retina as contrasted with the periphery. 



It is generally believed that movements are better seen peripherally 

 than centrally. The situation with regard to the objective {i.e., physi- 



