VISUAL ANALOGUES 177 



closing them, the composite image is analysed alternately 

 into its component parts, and thus we are enabled to 

 read better with eyes shut than open. 



This period of alternation is modified by age and 

 by the condition of the eye. It is, generally speaking, 

 shorter in youth. I have seen it vary in different indi- 

 viduals from 1" to 10" or more. About 4" is the most 

 usual. With the same individual, again, the period is 

 somewhat modified by previous conditions of rest or 

 activity. Very early in the morning, after sleep, it is 

 at its shortest. I give below a set of readings given by 

 an observer : 



Period Period 



8 A.M 3" 6 P.M 5-4" 



12 noon 4" 9 5-6" 



3 P.M 5" 11 6-5" 



Again, if one eye be cooled and the other warmed, 

 the retinal oscillation in one eye is quicker than in the 

 other. The quicker oscillation overtakes the slower, 

 and we obtain the curious phenomenon of ' visual 

 beats.' 



After-images and their revival. In the experiment 

 with the stereoscope and the design of the cross, the 

 after-images of the cross seen with the eyes closed are 

 at first very distinct so distinct that any unevenness 

 at the edges of the slanting cuts in the design can be 

 distinctly made out. There can thus be no doubt of 

 the ' objective ' nature of the strain impression on the 

 retina, which on the cessation of direct stimulus of 

 light gives rise to after- oscillation with the concomitant 

 visual recurrence. This recurrence may therefore be 

 taken as a proof of the physical strain produced on the 



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