STROBOSCOPIC VISION IN ANIMALS 363 



fishes, it probably exists all the way up to the dog and man. There 

 appears no positive reason why it should have been eliminated by any par- 

 ticular group, though it must be admitted that its usefulness is obscure. 



The scanty experimental work indicates that stroboscopic movement 

 is seen by animals, as by man, as practically indistinguishable from real 

 movement of an equally luminous object over the same distance in the 

 same time. If further work is done along this line, and shows this rule 

 of 'identity' to hold firmly in case after case, we shall have a powerful 

 experimental tool for exploring animal capacity for real-movement per- 

 ception; for, as a matter of technical convenience, the ^-phenomenon 

 is more easily presented to an animal than is an equivalent real move- 

 ment. The apparatus is simple, and the control of such factors as accom- 

 panying noise is much easier. Variations are more easily introduced, for 

 it is far simpler to space two lights farther apart than to alter machinery 

 which moves one light back and forth. 



A few years ago. Mile. Gaffron demonstrated stroboscopic vision in 

 two fishes, Phoxinus Icevis and Gasterosteus aculeatus, though with the 

 same technique she could find no evidence of it in various insects. The 

 dish containing the fish was surrounded by a cylinder capable of rotation 

 on a vertical axis and bearing vertical stripes on its inner surface. Rotated 

 slowly in either direction, the cylinder naturally evoked the optomotor 

 reaction — the fishes either swimming around with it or turning into a 

 radial position in which the eyes alone followed the movement. When 

 the cylinder was now illuminated intermittently, at different frequencies, 

 the fish responded to it as if it were motionless, or turning in its actual 

 direction, or turning in the opposite of its real direction, depending upon 

 the timing of the flashes relative to the positions of the stripes. Interest- 

 ingly enough, the apparent motionlessness, forward movement, and 

 apparent reversal of the cylinder were each seen by human observers 

 under the same conditions as by the fishes. 



Almost simultaneously, von Schiller published his researches on the 

 stroboscopic vision of Phoxinus. He used a form of the (^-phenomenon 

 situation in which two white squares, set one above the other with a 

 certain separation, could be revealed at one end of the aquarium for vari- 

 ous periods and in succession at various time intervals. The fishes were 

 initially trained positive to an actual upward movement of a similar 

 white square, which was made a signal for food. Presented then with the 

 ^-phenomenon, with the object, duration, and distance all identical with 

 the real movement to which they had been trained, the fishes responded 



