meant, and what is the drift of the experiments in point. And in considering the centripetal aspect of nervous processes, we may with advantage appropriate to the three-level scheme the three terms impression, sensation, perception-using 'impression' as the lowestlevel term, 'sensation' as the middle-level term, 'perception' as the highest-level term; and taking impression to denote an effect that does not reach consciousness, sensation (Empfindung) to denote a felt impression, perception (Wahrnehmung) to denote a sensation in relation with its felt circumstances, i.e. the group of associated sensations that complete the mental picture or representation of the moment (Vorstellung), forming what may be designated as a field of attention. The reaction time is the interval between the application of a stimulus and the responsive signal indicating that the stimulus has been felt.' This interval is conveniently measured by arranging an electro-magnet to mark on a revolving cylinder (1) the moment when a tactile, auditory, or visual stimulus is applied, and (2) the signalling movement by which the person experimented upon indicates that he has felt the stimulus. The reaction time varies in different subjects, with different modes of stimulation, and with different degrees of attention and of health, between ten and twenty hundredths of a second. Average values of the reaction time are: The total reaction time is composed of (1) the time occupied in conduction up sensory and down motor nerves and (2) the time occupied in the central elaboration, during which entering impression gives rise to outgoing impulse. Thus, in the simplest case, where the skin of the hand is stimulated by an induction shock, and the signal given by the same or by the opposite hand, the interval (say '15") between stimulus and signal is made up of the time of conduction along sensory nerve (say '02") + the time of conduction along motor nerve and the muscular latency (say 03") + the time of cerebral elaboration, i.e. perception of sensation and formation of volition (say '10"). It is the last of these three factors that varies most-with idiosyncrasy, attention, health, &c. and it is partly on this account, partly because the sensibility differs at different parts of the skin, that measurements of the reaction time with cutaneous stimulation near and far from the head give no admissible data for estimating rate of conduction along sensory nerves. The reaction time is shorter when attention is concentrated upon the intended movement than when it is concentrated upon the expected stimulus; this signifies that the formation of volition involves more labour than the perception of sensation, the preparatory influence of attention being more effective if bestowed upon the outgoing than upon the incoming event. With regard to individuality, it is not the case that persons reputed quick and wide-awake' have a shorter reaction time than persons of an apparently sleepy and phlegmatic temperament. Quick' people often have a long reaction time; 'slow' people often have a short reaction time; the relation, although common, is, however, not constant enough to constitute the rule. Experiments can easily be devised so as to yield an approximate estimate of the shortest time required to discriminate between two sensations (discrimination time), and of the shortest time required to determine an act of volition (volition time). For instance, the hand of a person, with bandaged eyes, on whom the simple reaction time to touch has been determined to be, say, 15", is stimulated on the little finger or on the thumb, with the understanding that only one of these stimuli is to be signalled; the reaction time is now found to be, say, 17", from which the conclusion is drawn that 02" was the discrimination time, i.e. that required to distinguish between the two different sensations. Or the experiment is conducted with two signals, on the understanding that one signal is to be used when the little finger is touched, and the other when the thumb is touched; the reaction time under these conditions being found to be 20", is considered to be the sum of 15", the simple reaction time, +02", the discrimination time, + '03", the volition time. The experiments may be still further complicated NN in a variety of ways, to measure the time of recognition of letters, words, simple ideas, &c., and are then distinguished by the title 'psychometric.' Stimulation and sensation. The Weber-Fechner law.--Our measure of sensation being necessarily subjective, consists at best in the perception of the least observable difference between two sensations. We assume that this least observable difference or differential sensation is a constant sensory quantity, or one sensation unit; e.g. the differential sensation between two weights of 170 and 180 grammes, and the differential sensation between two weights of 1,700 and 1,800 grammes, are found by experiment to be 'least observable differences that is to say, equal sensations. Each such sensation is then assumed to be one unit, and, in accordance with Fechner's exposition, the assumption includes the differential sensation between zero and the smallest appreciable quantity of any stimulus-this minimum perceptible sensation being, in fact, the smallest observable difference between zero and something. This assumption is the basis of psychophysics. Within certain limits, the strength of sensation increases with the strength of stimulation, and the relation is such that each equal increment of sensation requires an increasing increment of stimulation; e.g. a given sound of intensity = 1, producing a sensation = 1, must be increased to an intensity before any difference becomes perceptible, to an intensity xor (1)2 before a second difference is perceptible, to an intensity=xor (1) for a third difference, to an intensity = x× xor (4) for a fourth difference, &c.2 Thus the relative magnitudes of stimulus and of sensation will be expressed by the following numbers, or graphically by fig. 291: = = 'Nothing can be perceived otherwise than by comparison with something else; the most elementary sensation is the resultant of a ratio between two sensificatory terms, a and b, and not that of a single term, a or b. Something is perceived in comparison with nothing or with something else, and the resultant of the ratio in consciousness is a sensation. The highest forms of sensation are equally reducible to a ratio between two terms, each of which may, however, be a highly compound ratio. 2 Sensation increases arithmetically, while stimulation increases geometrically; the equation to the curve, expressing their ratio, is x = a; i.e. the curve is logarithmic, and the sensation y varies as the logarithm of the stimulus x, since loga ; this is Weber's law. Fechner's formula is y = k (log x-log x), where y is the sensation, k an experimental constant (differing with the nature of the sense-organ), ≈ the stimulus, and x, the liminal intensity (reizschwelle). = Thus we may conceive that any sensation mounts from zero to a maximum by a series of equal units, being the least perceptible differences between its successive magnitudes, beginning from the first magnitude or minimum perceptible difference between nothing and something. In fig. 291, where sensory units are represented along the ordinate and stimulation units. along the abscissa, we see the progressive increase of the stimulation increments required for each successive step of sensation increase. Otherwise expressed, and without the assumption that sensation units may be treated as of constant magnitude, the relation may be put thus: Within a certain range the smallest perceptible difference of stimulation is a constant fraction of the original stimulation. This fraction varies with different sense-organs; thus, for light the smallest perceptible difference is, for weight it is, for sound it is. This relation, known as Weber's law, is a phenomenon of everyday experience, affecting every province of human feeling, setting a limit to pains and pleasures, yet permitting the appreciation of differences within a very extensive range of stimulation intensity. Referring to the curve given above, we see how rapidly a maximum of sensation is approached with increasing stimulation intensity, and a glance at the table on p. 549 shows us within what a wide range of light intensity we habitually live. The most convenient stimulus with which to follow Weber's law experimentally is that of light, and we shall describe how this may be done with very simple appliances, and in recollection of the physical law that the intensity of light varies inversely as the square of its distance. Experiment.-Place a lighted candle at the distance of 1 foot in front of a vertical white screen in a darkened room. Let the illumination of the white screen represent unit intensity, or 1. Take a second candle and any convenient opaque body, so as to obtain a shadow in the light from the second candle; move this candle away until the shadow is just perceptible or just imperceptible, and measure the distance, which is, say, 10 feet. From these data the minimum perceptible difference is easily calculated. The intensity of light from the first candle is 1, from the second candle or the shadow on the 1 102 screen has an illumination from the first candle of 1, the unshaded portion has an illumination from both candles of 1+, i.e. the smallest observable difference is th of the original candle-light. I Repeat the experiment with a light of two candles 1 foot from the screen; the distance at which a third candle will give a shadow will be about 7 feet; the intensity of light in the just perceptible shadow is 2, and in the unshaded part 2+5; i.e.' the smallest observable difference is, or about Repeat the experiment with a lamp replacing the two candles. Let us say that a shadow was just perceptible from a candle at 9 feet when the lamp was at 4 feet. The illumination of the candle at 9 feet is th of the illumination of a candle at 1 foot. Assuming as a datum the previously-determined fraction, 15, as representing the minimum perceptible difference, we have the illuminating power of the lamp at 4 100 of a candle at 1 foot 10016 of a candle at 4 feet; i.e. the lamp-light is equal to that of nearly 20 candles. feet = 81 = ST On the same principle, we may approximately determine in terms of candle-power the intensity of diffuse daylight or of moonlight by observing the maximum distance at which a standard candle can cast a shadow on a white screen receiving daylight or moonlight. If in the first case that maximum distance were 2 feet, and in the second 110 feet, we should know that the value of daylight at the time was] × 100, or 25 candles; of moonlight, 2100 × 100, or TT candle-assuming, as before, that the minimum perceptible difference was 1 per 100. |