Page:The New International Encyclopædia 1st ed. v. 06.djvu/335

DISCRIMINATION. 287 DISCRIMINATION. the report of their difference. . . . But we also use it to iudieate our experience of like contents, and our report of their likeness" (Kiilpe). The differential sensitivity would, then, be exercised if we were called upon to jud-ie of tlie likeness or difference of two reds, or of the a of two violins. In such cases, we (a) experience the colors and tones in the same or in different ways: they make the same or a different impression upon our mind; while we then (b) formulate this experience in words, saying "like' or "different," as introspection dic- tates. The former process, of introspective dis- crimination, is termed the direct, the latter, of remlering the experience in words, the indirect differential sensitivity. There are, then, two problems before us. We must ask, first, as to the conditions under which we are able to discrimi- nate, and the limits within which discrimina- tion is valid: and, second, as to the adequacy of lamiuage to reflect or reproduce the results of introspection. ( 1 ) A very little consideration sirffices to show that an accurate comparison of sense-contents cannot be undertaken in any and every case of difference : but that certain favorable conditions are necessary to a reliable judgment. Suppose, e.g. that we have two reds before us — a saturated red. and a reddish gi-ay of the same brightness. (See S.iTVE-^Tiox.) It is impossible, with all imaginable care in comparison, to assert that the saturated red is three or four or any definite numtier of times as red as the less saturated color. The one impression is that of a 'good' red: the other that of a dull or gray red: more we cannot say. Or suppose that three pressures are given, a light, a moderate, and a heavy. It is impossible to assert with any confidence that the moderate pressure is three times as much heavier than the light as the hea^-y is than the moderate: we can say that the former difference is 'a good deal greater' than the latter, but that is all. Xow experiment has proved that the conditions favorable to exact discrimination are realized in two. and only in two. instances; and it is. accordingly, with these that investigators of the differential sensitivity have occupied them- selves. The first instance is that of 'difference determination.' the second is that of 'difference comparison.' In the former procedure, the psy- chologist aims to determine the 'just notice.ible difference' between two like contents ; in the lat- ter, he seeks to effect a subjective equality of stimulus-differences. An illustration of differ- ence determination appears under the title Au- dition (q.v.) : it is found that two tones are disoriminable, in the middle region of the music- al *cale. if their pitch-numbers differ by 0.2 of a vibration. This value 0.2 is the just notice- able difference of tonal quality. As illustrations of difference comparison, we may take the follow- ing: given two tones, it is require^ to find a tone which lies, for sensation, midway between them: and. given a black and a white, it is re- quired to find a gray which appears to be equi- distant from both. Let the tones have, e.g. the pitch-numbers 200 and 400; then the middle tone has (approximately) the pitch-number 300; the stimulus-difference .300 200 is equal, in sen- sation, to the stimulus-difference 400-300. In other words, we have the required middle tone when we have the arithmrtimj mean between the two tonal stimuli. With brightnesses the case is diflercnt. Let the unit of black have the photo- inetric value 1, and the unit of white the photo- metric value 00; so that a white disk (300°) stands to a black disk as 300 X 00 to 300 X 1, or as 21,000 to 300. The middle gray is not the arithmetical mean, a disk of 180° white and 180° black (value 10,!I80). Tliis mixture appears much too light. The middle point is found rather with a disk of some 41° of wliite and 31!)° of black; i.e. with a stimulus who^e photometric value is appro.ximalely 2788. the geometrical mean between the limiting black and white. It is clear, then, that the course of the dili'erential sensitivity is not the same for tones and for brightnesses. By an extension of experiment to the qualities of the other senses, and to the sensa- tion-attributes other than quality (see Sensa- tion), various laws of the differential sensitivity have been established. ( For an example, see Weber's L.w.) A valid determination of these laws is possible only on the condition that the sources of experimental error are strictly con- trolled by the investigator. Attention, expecta- tion, and practice must l)e at their best : habit- uation and fatigue musit be avoided (see these terms). The 'constant errors' of time and space must also be ruled out ; as many experiments must be taken in the time-order ab as in the order hn, and as many with stimulus a to the right as with the same stimulus to the left ; and the results of the double series averaged. So far we have spoken of the magnitude of the differential sensitivity. This may be ex- pressed in two ways, absolutely and relatively. If, e.g. I can just distinguish a light of 50-candle power from one of .50.o-candle power, the value .5 gives a measure of the absolute, and the value .5/50 or I/lOO a measure of the relative differ- ential sensitivity. But we can also measure the delicaeii of the discriminative function. An ob- server might, in one test, distinguish between lights of 50 and 50.2, while in the next succeed- ing he .could distinguish only between 50 and 50.8. Here the magnitude of the differential sensitivity, as averaged from the two tests, is .5 ; but the obser'er is evidently less delicate of discrimination than another who should give the values 50.4 and 50.6 in successive trials. Xot only the average, but tlie fluctuation of that average, must therefore be considered in a gen- eral measure of discrimination. (2) We turn to the question of the indirect differential sensitivity. And we must note, in the first place, that the importance of language as an instrument of psychology can hardly be over- estimated. The flexibility and ready variation of the spoken sentence and the permanence of the written record are verv great advantages. Still, language is discrete, while the stream of consciousness is continuous ; so that, at the best, we are seeking, so to say, to reproduce a fresco bv a mosaic. Moreover, the relation of the con- tents of consciousness to the words that describe them is not constant; while some experiences call up habitual and familiar phrases, others baffle us. cause us to pick and choose our terms, and to halt for the right expressions. We can- not, then, take it for trranted that discrimination and the report of discrimination always run parallel, that the 'good observer' is necessarily a 'good reporter.' The experimental psycholo- gist must receive a special training in the use of words; he must be taught to clothe his intro-