THE SENSE ORGANS 485 



is the eye ; by this organ we can appreciate differences of one one-hundredth 

 to one one-hundred-and-sixty-seventh in the total illumination. 



FECHNER'S LAW gives the result of an attempt to state Weber's law 

 in mathematical terms. It states that the sensation varies as the natural 

 logarithm of the stimulus. This relationship is shown diagrammatically in 

 Fig. 242. 



In view of the fact however that Weber's law holds good only between certain 

 limits, not much practical value can be attached to such a mathematical expression. 

 Moreover Fechner's calculation is based on the unprovable and unjustifiable assump- 

 tion that, within the limits of applicability of Weber's law, the smallest appreciable 

 increase in sensation is always the same, i.e. that the increased sensation which is evoked 

 by the addition of 6 grammes to a weight of 100 grammes is identical with the increased 

 sensation called forth by adding 60 grammes to an initial weight of 1000 grammes. 

 Such an assumption does not, as a matter of fact, agree with our own experience ; and 

 it is probably premature here, as in many other departments of biology, to attempt 

 to include the complex of variable phenomena presented by animal functions within 

 the Procrustean bed of a mathematical formula. 



