DISCRIMINATION, SENSIBLE, the awareness by a person or an animal of a difference between two very similar sensations or sensory excitations. The capacity for sensible dis crimination is called differential sensitivity. The sensory differ ence discriminated may be one of quality, as the hue of colours or the pitch of tones, or of the intensity, extent or, less often, duration or some other aspect of the sensory impression. The just noticeable difference, often called the "j.n.d.," between the stimuli of two sensations, when statistically determined by the methods of psychophysics (q.v.), is the differential threshold (limen). The corresponding sensed difference was thought by Fechner to constitute a unit for the measurement of sensation, but scientific psychology has rejected this view on the ground that all j.n.d.'s cannot be proved to be equal. It was believed for a time that a sensory scale varied by discrete steps, each quantum of sensation being a j.n.d., but it is now thought that sensation is a continuous function of its stimulus for the following reason. If we begin with a tonal pitch corresponding to 435 vibrations per second, we may have to increase the frequency to 435.5 before a difference in pitch is sensed. In such a case the differential limen is 0.5 vibrations per second. However, if we begin with a stimulus of 435.3 then the sensed difference would be expected to occur at 435.8. It thus appears that there are no fixed critical points in the tonal scale, and the conclusion is that there is an infinite number of sensations in the scale, but that two very simi lar sensations given together can not be discriminated from each other unless they differ by an amount that is, more often than not, greater than the limen.
On the side of quality there has been a great deal of interest in determining how many different sensations can be discriminated from one another under favourable conditions. Approximate re sults indicate that the normal human eye can discriminate about 150 hues in the psychological spectrum from red to yellow to green to blue and back, through the purples, to red again; that it can discriminate about Boo greys between black and white; and that, for the intermediate illuminations, there are perhaps an odd hundred discriminable saturations in the fading out of the best saturated colours into grey. Light and dark colours give fewer . discriminable degrees of saturation, but the total tale of discrim inably different visual sensations is probably as great as two million. Between the highest and the lowest audible tone there would seem to be about I0,000 discriminably different pitches, of which music seldom uses more than 400.
The facts of the sensible discrimination of intensity are the basis of Weber's Law, which states that the least discriminable difference, in terms of the stimulus, is always proportional to the total magnitude of the stimulus. Thus it is possible, in as far as Weber's Law holds, to state the differential intensive limen as a fraction. For illumination it is thought to be about that is to say, a sensed difference is created when the illumination is in creased by of its amount ; e.g., from i oo to 1 o I . For the sensible discrimination of lifted weights the fraction is about ; for light pressure upon the skin it is for tone it is about and for intensity of smell of the order of a to 3.
(E. G. BoR.)