191.3.] Sensations and the Theory of Forced Vibrations. 503 



Fechner's original papers, and do not know what was his opinion on the 

 point, but I consider that there is strong evidence that the shunt-factor is a 

 function of the retina rather than of the brain. 



The form of the curves of colour-sensation during very bright light may 

 now be discussed. The supply dn/dt of sensitive material is limited by the 

 rate of metabolism possible to the tissues, but the rate at which it is 

 converted into exciting substances, dx/dt, depends on the intensity of the 

 light. Therefore long exposure to a bright light will make the concentra- 

 tion of the sensitive material, and consequently also the molecular resistance, 

 tend to a minimum. But when friction is low the amplitude of the foiled 

 vibration is slightly greater whatever be the period of the impressed vibration, 

 and very much greater as this approaches the period natural to the 

 resonator, so that the curve develops a very sharp central spike. This 

 spike cannot be reproduced in the corresponding curve of colour-sensation 

 because of the separation of the opto-chemical from the physiological functions 

 of the eye. The active material, however concentrated, cannot do more than 

 excite a maximal response. 



But the excess of active material may excite the protective arrangement, 

 or shunt function, to stronger action, so that the maximal response may only 

 send through to the central organ a quite moderate sensation. 



This state of things is illustrated in fig. 3, where the dotted line represents 

 the first beginnings of artificial green-blindness, before the slit has been 

 opened wide enough to dazzle the eye. The response is well within the limits 

 of possible sensation, so that there is no truncation of the curve of resonance, 

 although by the shunt-factor its ordinates are reduced to one quarter of their 

 normal value. 



It will be noted that the effect is to lessen the apparent extent of the green 

 sensation. And this is precisely what happens. The red sensation on the 

 one side, and the blue sensation on the other, encroach on the green. A 

 powerful light is not required to show this. My method of testing for colour- 

 blindness is based upon it. After looking at the 5-lines in a spectrum of quite 

 ordinary intensity for 30 seconds the boundary between red and green is found 

 to have shifted from 100 to -100 A.U. nearer the green, if the observer possesses 

 a normal green sensation. 



The height of the apex is inversely proportional to the coefficient of friction. 

 When, therefore, this is further reduced by the action of a strong light, a 

 stage is reached when the curve is truncated. For the height of the apex of 

 the resonance curve is limited because in the first place dx/dt, the rate of 

 production of the exciting substance, cannot exceed dn/dt, the rate of secretion 

 of the sensitive visual material, and in the second place because de/dt, the 



