68 
C. E. Seashore. 
terms of each other, for on this theory we may add the illusion of A to the 
illusion of B and take the sum of these as the expression of the illusion of 
either A or B in terms of the other. Thus 
JA B-{K+K') 
JB~ B 
and ^=^5+*!). 
JA A 
The error involved by substituting the actual figures is 5% for the first 
equation and 7 % for the second. Although these are extreme cases the 
error of the substitution is no greater than that found when the mean, C, 
was used as a standard. Therefore, within obvious limits, we are justified 
in choosing the standard of any convenient size in measurements like 
these. 
Just as no one now claims an exact mathematical conformity for 
Weber’s law in any sense, we must construe the above formula liberally. 
There may be some more determinable factors that must be taken into 
it ; we can never hope to determine and control all such factors. Judg¬ 
ments as to the validity of the law have heretofore been made largely upon 
experiments that involved the illusion here discussed, or similar ones, and 
the variations caused by them have been counted as discrepancies in the 
law. The above data at least justify us in assuming this relation of Weber’s 
law to illusions as a working hypothesis. It promises not only the same 
degree of comformity as the law has had on the old theory, but also an 
extension of it both in the degree of conformity and the range of its ap¬ 
plicability.