706 THE EYE IN EVOLUTION 



a retinal element, once stimulated, cannot react quickly to a new 

 stimulus ; an image moving across such a retina will therefore 

 appear as a blurred streak and not as a clearly defined pattern. The 

 persistence-time can be studied by the well-known method of flicker, 

 and can also be determined objectively by studying the electrical 

 reactions of the retina to intermittent stimulation. We have already 

 seen that great differences exist in this respect between the " fast eyes " 

 of swiftly moving diurnal insects and the " slow eyes " of more sluggish 

 nocturnal types. Similarly among Vertebrates the persistence-time 

 is shortest in rapidly moving animals of diurnal habit. Both a fine 

 retinal " grain " and a short persistence-time are therefore associated 

 with the mechanism required for good visual acuity and the apprecia- 

 tion of movement. 



The limits of the perception of movement in Vertebrates have 

 not received much study. Boulet (1953-54) found that if several 

 perch (Perca fluviatilis) were confronted by a moving sphere in con- 

 trolled conditions to excite the optomotor reaction, half the fish 

 responded with eye movements when the angular velocity was 12° per 

 sec, and all of them when it was between 14° and 26° per sec. ; move- 

 ments quicker than 78° per sec. excited no response and were probably 

 not perceived as such. This compares poorly with the performance of 

 the human fovea where the minimum angular displacement perceived 

 is from 6 to 10 sees, of arc and the upper perceivable limit of speed 

 corresponds to an angular velocity between 140° and 350° per sec. 



The perception of movement is, of course, only relative. Beebe 

 (1934) brought this out well by his observations on the conduct of fish 

 in his oceanographic studies. When standing on the ocean floor, so 

 long as he stood motionless and erect he excited the attention and 

 curiosity of the surrounding fish, but if he rocked and swayed with the 

 current in keeping with the weeds of the sea-bottom, they paid no 

 attention to him and appeared not to see him. 



Animals appreciate stroboscopic movement in much the same way as we 

 do. Thus Gaffron (1934) found that if fish were contained in a tank surrounded 

 by a revolving striped drum illuminated intermittently, they reacted as if the 

 drum were stationary or were turning in the actual direction of motion or in the 

 opposite direction depending on the frequency of the illuminating light, the 

 response of the fish being precisely similar to her own (Gaffron's). Similarly, 

 von Schiller (1934), having trained the minnow, Phoxinus, to resiaond positively 

 to the upward movement of a white square at a definite speed in feeding experi- 

 ments, found that the same response could be elicited if two squares were 

 successively illuminated at time-intervals such that the stroboscopic movement 

 thus appreciated eorresjaonded to the real movement in the initial experiment. 

 In this connection Walls (1942) pointed out that the interest of the dog in motion 

 pictures and its complete indifference to still pictures is a demonstration that 

 to it also an appreciation of apparent movement corresponding to that of man 

 is a real perceptual experience. 



