This is a new science which you may read about in Terman's "The Measurement of Intelligence." By testing normal children, it is established that certain tasks can be performed at certain ages. A child of three can point to his eyes, his nose and his mouth; he can repeat a sentence of six syllables, and repeat two digits, and give his family name. Older children are asked to look at a picture and then tell what they saw; to note omissions in a picture, to arrange blocks according to their weight, to arrange words into sentences, to note absurdities in statements, to count backwards, and to make change. Children of fifteen are asked to interpret fables, to reverse the hands of a clock, and so on. Of course there are always variations; every child will be better at some kinds of tests than at others. But by having a wide variety, and taking the average, you establish a "mental age" for the child--which may be widely different from its physical age. You may find some whose minds have stopped growing altogether, and can only be made to grow by special methods of education. Enlightened communities are now conducting separate schools for defective children--replacing the old-fashioned schoolmaster who wore out birch-rods trying to force poor little wretches to learn what was beyond their power. In the same way psychology can be applied in industry, and in the detection of crime. Here, too, there is a vast amount of "fake," but also the beginning of a science. Our laws do not as yet permit the use of automatic writing and the hypnotic trance in the investigation of crime, but they have sometimes permitted some of the simpler tests, for example, those of memory association. The examiner prepares a list of a hundred names of objects, and reads those names one after another, and asks the person he is investigating to name the first thing which is suggested to him by each word in turn. "Engine" will suggest "steam," or perhaps it will suggest "train"; "coat" will suggest "trousers," or perhaps it will suggest "pocket," and so on. The examiner holds a stop-watch, and notes what fraction of a second each one of these reactions takes. The ordinary man, who is not trying to conceal anything, will give all his associations promptly, and the reaction times will be approximately alike. But suppose the man has just murdered somebody with an axe, and buried the body in a cellar with a fire shovel, and taken a pocketbook, and a watch, and a locket, and a number of various objects, and climbed out of the cellar window by breaking the glass; and now suppose that in his list of a hundred objects the psychologist introduces unexpectedly a number of these things. In each case the first memory association of the criminal will be one which he does not wish to give. He will have to find another, and that inevitably takes time. One or two such delays might be accidental; but if every time there is any suggestion of the murder, or the method or scene of