SOME SENSORY MEASUREMENTS
It is neither feasible nor desirable to introduce here a detailed account of the methods and instruments required for accurate measurement of sensory capacity. Such an account must necessarily be both technical and voluminous, and is given properly in a laboratory course: the technique of accurate measurement can be acquired only by a laborious process under competent supervision. There are many pitfalls for the unwary, and an extensive range of cautions must be observed; so that it is impossible for the untrained worker to obtain reliable results, except for clinical purposes where comparatively rough measurements will suffice. It is possible, for example, to diagnose a grave defect of vision or of audition by the rough methods commonly employed in medical practice; but the lesser defects, even some of those which may have serious practical consequences, and the differences in sensitivity of "normal" individuals, can be measured only by the refined methods of the psychological laboratory.
The methods and technique of accurate measurement include not only a precise instrumental technique, and exact methods of presenting stimuli with proper gradations and order, but include also the proper time arrangement; the use of carefully planned warning signals; and various detailed provisions to secure uniformity of attention, lack of bias, and absence of emotional disturbance on the part of the reactor.
In certain cases, it is important and possible to make determinations in which small units of measurement are not demanded, and in which the reactor is not required to determine just perceptible and just imperceptible values. Such determinations are called tests; and although in each case certain details of technique