1913.] Sensations and the Theory of Forced Vibrations. 505 



indicate that it depends in some way upon contractile movements within the 

 tissues. It would be quite easy to imagine an arrangement whereby a 

 sensitive point might be dipped into or drawn out of a swarm of exciting 

 molecules set free by the action of light on a surface close to it. The actual 

 range necessary for such a movement to be effective would be extremely 

 small, so that it might easily escape notice. Each retinal element would act 

 independently of the rest, the protection from over-stimulation would 

 ■obviously be complete, and spasmodic action could easily take place. 



Areal induction, by which the illumination of one portion of the retina 

 affects the surrounding areas, might result from secondary cross-connections 

 between the retinal elements. 



Note on the Laws of Weber and of Fechner. 



The question arises whether the idea of a shunt factor, by which all 

 sensations are reduced in the same ratio, is compatible with the received view 

 that the light-sensation varies according to the logarithm of the stimulus. 



Weber's law is the statement of an experimental fact, viz., that " the 

 smallest perceptible difference of luminosity is a constant fraction of the 

 whole intensity of the light." And this is nearly true, though, according to 

 several observers, not accurately true, over a considerable range. 



Fechner's law is on an entirely different footing. It is based on mathe- 

 matical theory, and claims to establish a numerical scale of sensation : — 

 " The intensity of the sensation varies as the logarithm of the stimulus." 



If the sensation of light were a continuous function of one variable from 

 subliminal threshold to maximum, there could be no two opinion's as to the 

 validity of Fechner's law when "Weber's law is taken for granted. It is a 

 simple, obvious, elementary application of the calculus. But the sensation of 

 light is a function of at least three variables, viz., the production by light 

 of the exciting substance, the stimulation by this of the end-organs, and the 

 regulation by the shunt factor of the strength of the resulting sensation. 

 Moreover, the perception of a difference between two sensations, which is 

 necessarily implied in the statement of Weber's law, involves the judgments 

 of the central oi'gan, thus introducing a possible fourth variable. Under such 

 conditions we are not warranted in ascribing to Fechner's law the cogency of 

 a result obtained by the calculus as physicists use it. 



The experiments of Ebbinghaus quoted by Helmholtz* go to prove that 

 Fechner's law fails within a quite moderate range of intensities. My own 

 experiments extending over the last three years confirm this new, and bring 

 out, in addition, a point of considerable interest. 



* Helmholtz, ' Physiol. Optik,' 2nd ed., p. 392. 



