1902.] 



Contributions to the Study of Flicker. 



315 



the rotating disc ; the greater this is, the more difficult is it to see the 

 nicker, hence in the experiments recounted in this paper, the disc was 

 viewed at the same distance. The distance chosen was that of most 

 distinct vision, and though constant for each observer, differed slightly 

 for different observers. 



(3.) A black velvet mask was worn, both to avoid any possible com- 

 plication produced by light reflected from the disc on to the observer's 

 face, and thence either into the observer's eye obliquely, or back again 

 on to the disc. 



(4.) To determine the exact moment when all trace of nicker has 

 vanished requires a considerable amount of practice : but is made 

 far easier than it would otherwise be by the fact that nicker is much 

 more easily seen by averted vision than by direct, so that if the eye 

 be fixed upon the axis of the rotating disc, the nicker will be seen 

 to disappear in the region immediately around the axis before it 

 disappears from the parts of the disc near its circumference ; with 

 practice this makes it possible to time the appearance of the flicker 

 with very fair accuracy. 



It seems a somewhat remarkable fact that although the perception 

 of colour and form so rapidly deteriorate in passing from the fovea 

 centralis towards the peripheral portions of the retina, the retina's 

 sensitiveness to flicker should, on the contrary, increase. Inasmuch as 

 the perception of any motion is analogous to the perception of flicker, 

 for in both cases the perception is caused by a change in the stimulus 

 of certain retinal elements, it follows that the retina is increasingly 

 sensitive to the motion of objects as their images on the retina lie 

 nearer to its periphery. May not this be due to the fact that it has 

 always concerned mankind to notice motion before any other quality in 

 matter, and especially motion of objects near, the limit of his field of 

 view 1 When once this motion has been perceived, the man turns his 

 eye so that the image of the moving object is brought to the fovea, 

 where its colour and form can . be appreciated ; it seems, therefore, a 

 natural arrangement that the attention of an animal should, in the 

 first instance, be arrested by the sensitiveness of the retina to the 

 motion of an object which is not the object of direct vision at the 

 moment its motion is perceived. 



(5.) In timing the disappearance of flicker, it is not only a good 

 plan to fix the eye on the axis of the disc, as has already been said, but 

 it is necessary to keep the gaze steadily fixed on the axis, for when 

 flicker has just disappeared to direct and steady gazing, the smallest 

 movement of the e3^e, either obliquely across, or round the disc in the 

 direction of rotation will reveal the flicker momentarily, or for as long- 

 as the motion of the eye lasts. The reason for the reappearance of 

 the flicker scarcely requires explanation ; it is evident that any motion 

 of the eye such as that mentioned above will retard the rate of 



