in the position of details to
variations which keep the same motive or the same general arrangement,
but after all make the card strikingly different. The first step is to
determine for each pair the degree of similarity, on a percentage
basis. To overcome mere arbitrariness, we ask thirty to forty educated
persons to express the similarity value, calling identical postal
cards 100 per cent and two postal cards as different as a colored
flower piece and a black picture of a street scene O. The average
value of these judgments is then considered as expressing the
objective degree of similarity between the two pictures of a pair.
After securing such standard values, we carry on the experiments in
the following form. Six different postal cards, for instance, are seen
on a black background through the opening of a shutter which is closed
after 5 seconds. The six may be made up of a landscape, a building, a
head, a genre scene, and so forth. After 20 seconds the same group of
postal cards is shown once more, except that one is replaced by a
similar one, instead of one church another church building, or instead
of a vase with roses a vase with pinks. If the substituted picture has
the average similarity value of 80 per cent and we make the experiment
with 10 persons, the substitution may be discovered by 7 persons and
remain unnoticed by 3. We can now easily vary every one of the factors
involved. If instead of 6 cards, we take 10, it may be that only 4 out
of 10 persons, instead of 7, will discover the substitution, while if
we take 4 cards instead of 6, perhaps 9 persons out of 10 will
recognize the difference under these otherwise equal conditions. Only
an especially careless observer will overlook it. But instead of
changing the number of objects, we may change the periods of exposure.
If we show the 6 cards only for 2 seconds instead of 5 seconds, the
number of those who recognize the difference may sink from 7 to 5 or
4, and if we make the time considerably longer, we shall of course
reach a point where all 10 will recognize the substitution. The same
holds true of the shortening or lengthening of the time-interval
between the two presentations. The third variable factor is the
similarity itself. If instead of one church, not another church, but a
theatre or a skyscraper is shown, that is, if the similarity value of
80 per cent sinks down to a similarity of 60 per cent or 50 per cent,
the number of those who recognize th