Thus, for example, by the use of highly sensitive instruments we are able to detect the quickened heart-beat, the shudder, and other evidences of emotion not otherwise discernible, but due to the deliberate presentation of the details and evidences of a crime. Though the subject may not himself be aware of the slightest physical expression of emotion, these signs of a disturbed mentality are unerringly revealed by the delicate instruments of the psychologist. [Sidenote: _The Factory Operative's Attention Power_] In some factories the operative is called upon to simultaneously keep watch over a large number of parts of a moving mechanism, and to note and quickly correct a disturbance in any part. Eye and ear must have a wide range, must be able to take account of a large number of operations widely separated in space. [Illustration: TESTING THE RANGE OF VISUAL ATTENTION. PRIVATE LABORATORY, SOCIETY OF APPLIED PSYCHOLOGY] For the scientific determination of the operative's range of visual attention, the "disc tachistoscope," shown facing page 106, may be used. This is a form of short-exposure apparatus. The essential idea is to furnish a field upon which the subject may for a moment fasten his attention, and then to substitute for this field another containing certain prepared test-material. This last field is exposed for but a brief instant and removed, and the subject is then called upon to report all that he has seen during the last exposure. Tests of this kind have demonstrated that the range of visual attention is a comparatively constant quantity with each individual, having but little relation to general ability or intelligence and being but little affected by practice. It matters not how painstaking the individual may be, he will fail in a test of this kind and at work of this kind if the type of attention that Nature gave him is unfitted for such an "expanded" watchfulness. Yet in any type of work requiring a focusing of the attention upon a minute operation so as to note nice discriminations and detect subtle differences, he might prove a most excellent worker. [Sidenote: _Kinds of Testing Apparatus_] The kind of apparatus, the method to be employed and the place for the experiment are all matters that vary with the conditions of the special problem. The apparatus may be simple and easily devised, or it may be intricate and the result of years of investigation and a large expenditure of money. If there seems to you to be anything impracticable in the employment of tests in the manner we have indicated, please remember that for many years those seeking employment as railroad engineers have been required to pass tests for color-blindness, tests just as truly psychological as any that we have here referred to and differing from them only in respect to the character and complexity of the qualities tested. [Sidenote: _Analysis of Different Callings_]