1913.] Sensations and the Theory of Forced Vibrations. 501 



some familiar outline, e.g. a cross. The phenomenon occurs not only with 

 the direct image but with its positive after-effect, the waxing and waning of 

 which was known long before the time of Thomas Young. 



When making my experiments on artificial temporary colour-blindness 

 I particularly noticed that the positive after-effect of a brilliant mono- 

 chromatic light Over a large retinal area showed nothing of this periodicity 

 but died out steadily and gradually. 



Putting together these facts I have suggested in my paper on " Areal 

 Induction,"* that they indicate the existence of a protective arrangement in 

 the retina by which the eye is shielded from the sudden effects of too strong 

 a light. I now submit that there is sufficient evidence to identify this protective 

 action with the negative after-effect of which it is a special ease, and that the 

 other phenomena to which I have just referred come under the same category. 



The negative after-effect is so generally referred to as synonymous with 

 fatigue that some reference to that aspect of the problem is imperative. The 

 sensitive materials being used up and exhausted, the eye is supposed to be 

 rendered locally less sensitive for a while. But the photo-chemical conditions 

 indicate a state of increased activity, for if the store a is exhausted, 

 dnjdt reaches its maximum, and during continuous steady illumination 

 dx/dt = dnjdt. 



But it is easy to show that during a bright summer day the quantity of 

 sensitive material used without causing any sensation of visual fatigue 

 greatly exceeds the consumption during the production under suitable con- 

 ditions of very considerable retinal "fatigue." The explanation, therefore, 

 must be sought among the physiological rather than the photo-chemical 

 conditions of the problem. 



We may consider a few examples. The following phenomenon is 

 instructive, and probably familiar to most people. On a rather misty 

 afternoon, on coming to the window, the eyes rest on a bright gap in the 

 clouds. At first it appears too bright to look at, and the after-image is very 

 black. But if we persist, after a few seconds the sun's disc appears sharply 

 defined in the midst of the dazzling light. 



The retina can hardly be regarded as fatigued in the sense of being 

 exhausted and having its activities impaired, since we may continue watching 

 until the eyes have become completely adapted to the more brilliant 

 illumination, and further details — spots on the sun — isolated wisps of cloud 

 in the clear space— become visible, which were at first hidden in the glare. 

 In the subdued light of the room the store n of sensitive material had 

 become large. 



* ' Eoy. Soc. Proc.,' vol. 69. p. 129. 



