A group of objects is observed for a definite time and after a definite interval another group of objects is offered for comparison.  This second group is identical with the first in all but one of the objects, and this is replaced by a similar one.  The question is how often this substitution will be noticed by the observers.  I may give in detail a characterization of the set of experiments in which we are at present engaged.  We are working with picture postal cards, using many hundred cards of different kinds, but for each one we have one or several similar cards.  As postal cards are generally manufactured in sets, it is not difficult to purchase pairs of pictures with any degree of similarity.  Two cards with Christmas trees, or two with Easter eggs, or two with football players, or two with forest landscapes, and so on, may differ all the way from a slight variation of color or a hardly noticeable change 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