e substitution will again become
larger; if, on the other hand, the substituted card shows the same
church, only from a slightly different angle, bringing the similarity
value up to 90 per cent or 95 per cent, the number of observers who
recognize the substitution may sink to 2 or 3. To make the experiments
reliable, it is also necessary frequently to mix in cases in which no
substitution at all is introduced.

If these experiments are varied sufficiently and a large mass of
material brought together, we must be able to secure definite formulae.
We may find that if the critical card appears among 6 cards, is shown
for 5 seconds, and the group is again exposed after 20 seconds, 80 per
cent of the subjects will recognize the substitution of a similar
card, if the degree of similarity is 30 per cent, but only 60 per cent
will recognize it if the degree of similarity is 70 per cent, and only
30 per cent will recognize it if the degree of similarity is 90 per
cent. These are entirely fictitious figures and are only to indicate
the principle. If such an exact formula were definitely discovered, we
should still be unable to say from mere psychological reasoning what
similarity value is legally permissible. If the rules against
infringement are interpreted in a very rigorous spirit, it may seem
desirable to prohibit imitations which are as little similar as those
postal cards which were graded as 40 per cent in our similarity scale,
and if the interpretation is a loose one, it may appear permissible to
have imitations on the market which are as strongly similar as our
postal cards graded at 80 per cent in our similarity scale. All this
would have to be left to the lawmakers and to the judges. But what we
would have gained is this. We could say: if our object exposed for 5
seconds in a group of 6 other objects is replaced after an interval of
20 seconds by an imitation and this change is recognized by 8 persons
among 10, the degree of similarity is 30 per cent and if it is
recognized by 3 out of 10 subjects, the degree of similarity is 90 per
cent. In short, from any percentage of subjects who under these
conditions discovered the substitution, we could determine the degree
of similarity, independent of any individual arbitrariness. If such
methods were accepted by the trade and the courts, it would only be
necessary, to agree on the percentage of similarity which ought to be
permitted, and all uncertainty would disappear. There