ADAPTATION; CENTER VS. PERIPHERY 355 



respectively, pure-cone and pure-rod retinae under their respective optimal 

 conditions; for there are mysterious mutual inhibitory effects of the rod- 

 and cone-mechanisms of a duplex retina. But, from the data on critical 

 frequencies at least, we may suggest that a diurnal animal has his best 

 movement-perception in the daytime while a more rod-rich, nocturnal, 

 animal sees movements better at night than by day. In pure or simplex 

 retinae, acuity-differences between the light-adapted and dark-adapted 

 conditions are probably slight as compared with the differences in visual 

 acuity between light-adapted duplex retinae and dark-adapted ones; so, in 

 both pure-cone and pure-rod animals, the adaptation-state probably influ- 

 ences movement-perception chiefly through its effect on fusion-frequency. 

 When both diurnal and nocturnal animals are adapted to the same 

 illumination however, we should expect the movement-perception of the 

 diurnal form to be always superior on the grounds of both the visual 

 acuity and persistence-time factors. 



Other factors militating against the periphery are its poor perform- 

 ances in the matter of discriminating hues and the discrimination of 

 intensities. Both of these capacities are involved in visual acuity in its 

 broad sense — that is, in the perception of pattern, and consequently of 

 the changes of pattern which result from movements. Farther and farther 

 peripherally, fewer and fewer hues, less and less saturated, are seen. 

 At least, this is true of man, and probably of all color-perceptive verte- 

 brates. And, the discrimination of intensities (and hence, of contours 

 between areas of different objective limiinosity) is much poorer periph- 

 erally than centrally, and poorer in dark-adaptation than in light-adap- 

 tation. As a consequence, it is found that movements are perceived 

 farther into the periphery, and more easily, when the background is 

 made brighter to give the object more contrast. 



Wavelength as such can also influence movement-perception, presum- 

 ably even in animals which have no color vision; for the critical fre- 

 quency differs for different hues of light. A Swedish railroad recently 

 found that certain red signals, which had to be seen as blinking, could 

 be so seen if they flashed 75 times per minute. Blue ones could be 

 allowed to flash only 20 times per minute, else there was danger of 

 fusion by the dark-adapted eye of the engineer. 



We cannot say very much about the basis of the saliency of move- 

 ments, even in human vision. The apparent rate of a movement is almost 

 twice as great if some stationary object is fixated, as when a moving 

 object is pursued by the eye. Probably this is due to the fact that in the 



