the chemical constitution of a trade commodity is examined.  Certainly the experiment establishes here conditions which are very different from those of practical life.  The customer who wants to buy a particular picture postal card which he saw once before and to whom the salesman offers a similar one, suggesting that it is the same, is facing only one card and not a group of six.  But in practical life the card which be has seen was not observed with the definite intention of keeping the memory picture in mind, and months may have passed since it was seen.  The memory picture which the customer has in his consciousness when he seeks the particular card is much weakened by this circumstance too.  We secure this weakening artificially by the arrangement of the experiment in placing the card in a group of six or ten and exposing them for a few seconds only.  The force of attention and the corresponding memory-value are by this distribution diminished in a definite degree in the case of every single card.

The investigation must include a careful study of the size of the groups, of the time-relations, of the percentage of correct answers, all under the point of view of greatest fitness for practical application.  In the Harvard laboratory the research has been carried on partly with such picture material, partly with word material, and partly with concrete objects.[54] Whatever the details of the outcome may be, we hope that the work will lead to results which may, indeed, make such a psychotechnical use possible.  Its principles and formulae might easily be adjusted to any marketable material.  As a matter of course, if in future the courts were ever to accept such psychological, experimental methods, it would be intolerable dilettantism if such experiments were carried on by lawyers and district attorneys.  It is as true of this economic legal question as of many other legal psychological problems that its introduction into the courtroom can become desirable only when psychological experts are engaged and called in the same way as chemical or medical experts are invited to the court.  On the other hand, there is surely not the slightest desire on the part of psychologists to be dragged into humiliating performances like those which not only handwriting experts, but even psychiatric specialists have had to undergo repeatedly in sensational court trials.  The day for the expert activity in the courtroom will came for the psychologist only when the country has attached the expert to the court and has eliminated the expert retained by the plaintiff or the defendant.  But this general practical question as to the position of the psychologist in the courtroom and as to the need of a psychological laboratory in connection with the courts would lead us too far aside.

XXIII

BUYING AND SELLING