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Page:Popular Science Monthly Volume 49.djvu/781

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A MEASURE OF MENTAL CAPACITY.

wise even a long personal acquaintance with the candidate could not afford, and to determine with considerable exactness the working power of individuals in simple mental tasks. The measure is afforded by determining the number of small, similar problems resolved by the subject in a given time—such, for example, as numbering letters, reading, the learning by heart of series of numbers or syllables, and the continuous addition of columns of numbers. In the last-mentioned method the person under trial is set to adding figures ranged one under another in a book printed expressly for that purpose, for a considerable time, without stopping—under some circumstances, for several hours. When the sum reaches a hundred, the hundred is simply carried on and added to the excess in units. A bell sounds every five minutes, when the candidate draws a line after the last-added number. At the end of the trial it is easy to determine how many numbers the person can add every five minutes.

The candidates—all of nearly equal degrees of advancement, and of about the same age—varied greatly in the speed of their execution, the more rapid ones adding two and a half times as many numbers in five minutes as the slower ones. This proves that facility in reckoning is largely peculiar to the individual. Accuracy, however, was not considered. If that had been brought in, some of the results might have been materially different.

It further appeared that the speed of the additions increased regularly with each effort, but not equally with the different subjects, so that it was possible sometimes for a slower calculator eventually to pass ahead of the next quicker one. This improvement in facility was, however, subject to limitation, and is less in each repetition—as, for example, twenty-five per cent from the first trial to the second; fifteen per cent from the second to the third; and about six per cent from the third to the fourth—till a point is finally reached when there is no further increase. This capacity for improvement through practice appears also to be an individual quality. The permanence of the acquisitions obtained through it has not been sufficiently investigated; but they seem in the end gradually to wear out, and the rapidity of the wearing-out process to vary with the persons.

Of an opposite character to this is the far more rapidly increasing effect of fatigue, which always causes a diminution of efficiency, however much it may at first be temporarily balanced by the improvement through exercise. When it has once gained the upper hand, a speedy and unintermitted decline of efficiency ensues. The time when this shall take place depends on the degree of capacity already reached, the personal peculiarity, and casual influences.