484 



NATURE 



{Sept. 28, 1876 



stimulus ; the result being that, so far as the retina is concerned, 

 it comes to the same thing whether an intense light acts for a 

 brief time, or a faint light for a longer time. 



Exhaustion of Nerve or Sensory Organ. 



This line of research has also made it possible to measure the 

 time required for exhausting a nerve or sensory organ. When, 

 for instance, a limited area of the retina has been stimulated for 

 a certain time, and the stimulus has been removed, the after 

 positive effect, due to increased excitation of the parts, disap- 

 pears, and is followed by a negative effect, due to temporary 

 diminution of the sensibility of the parts, in the form of what is 

 called the negative after-image. Suppose, for example, an area 

 of the retina be acted upon for a period of from five to ten 

 seconds, and the stimulus be then removed, the so-called positive 

 after-image vanishes quickly, and the negative after-image, fre- 

 quently of a complementary colour to that of the exciting cause, 

 appears, and lasts for a short time, gradually fading away as the 

 nervous parts recover from the effects of the stimulus. Similar 

 phenomena may be observed in studying the durations of sensa- 

 tions of tone, which I have frequently perceived in experiments 

 made by myself; but it is more difficult to identify, by descrip- 

 tion and designation, the after effects in the case of audition 

 than in the case of vision. Probably it may be found still more 

 difficult to notice these after sensations in the other senses, 

 although in all there is often the experience of a lingering feel- 

 ing alter the cause has been removed, which no doubt has its 

 place in those transient sensations which assist in filling up the 

 spaces, as it were, in our conscious life. 



In experiments upon a sensory organ, such as the retina, a 

 little consideration will show that it is almost impossible to as- 

 certain the effect of a stimulus upon a retina which has never 

 before been affected. This difficulty has been felt by all experi- 

 menters. Molecular action in such a structure has been in opera- 

 tion from the very beginning, and such action, if of sufficient 

 intensity, must produce a certain effect on the conducting tract, 

 and on the recipient centre. This effect, although of too weak 

 intensity to produce those changes which result in consciousness, 

 must be taken into account in the measurement of the intensity 

 and duration of sensory impressions. Thus the eye has a light 

 of its own due to changes in the retina, although this may never 

 be conscious to us as a luminous impression. This conception 

 of the state of matters in a terminal organ such as the retina, 

 when applied to actions going on in the brain, at once indicates 

 that similar actions, or rather that similar states of unrest, of 

 change, variation, and modification, are going on in these deeper 

 paits which may never result in consciousness, per se, but which 

 altogether may have an influence on our mental existence com- 

 parable to that of the feeble impressions constantly transmitted 

 to the cerebrum from the viscera, sometimes termed the internal 

 senses. 



Relation between Strength of Sensation and Magnitude of Stimulus. 



Having shown that sensory impressions are distinctly related 

 to time, the next advance made by physiologists was to prove 

 that there was a relation between the strength of the sen- 

 sation and the magnitude of the stimulus. Here there are diffi- 

 culties in explaining what is meant, because language fails. We 

 have r:o words to discriminate ideas which hitherto have related 

 to two distinct fields of knowledge — the objective and the sub- 

 jective. To speak of the strength or magnitude of a sensation 

 seems to be using terms applicable only in another region, and 

 quite inapplicable to psychological phenomena, although no one 

 has any doubt in distinguishing the intensity or magnitude of one 

 pain from that of another. There is no difficulty in understand- 

 ing the phrase-magnitude of tlie stimulus. A weight of ten 

 pounds is greater than that of one pound, light from ten candles 

 of equal size is more than that given out by one, and the tones 

 of a violin of equal pitch and quality, may vary in intensity 

 according to the pressure of the bow on the string. It is diffi- 

 cult, however, to obtain an absolute measurement of variations 

 in sensation, which is, of course, a subjective phenomenon. This 

 can only be done by varying the objective cause, by observing a 

 larye number of instances, and by expressing variations in the 

 subjective phenomenon in terms applied to variations in the 

 objective cause. If the average result obtained from a large num- 

 ber of instances indicate any ratio between the magnitude of the 

 stimulus and the subjective phenomenon, then we may conclude 

 that there is a relation between the two. 



This mode of inquiry, first originated by Prof. E. H. Weber 

 in his celebrated experiments on tactile impressions (and which 



were first introduced to notice in this country by Prof. Allen 

 Thomson), was afterwards carried out by his colleague Prof. 

 Fechner, and has been subsequently elaborated by Prof. Wundt. 

 It has led to various remarkable results, the chief of which are — 

 (i) That in the case of each sense there is an upper and a lower 

 limit, beyond which the amount of stimulus produces no appre- 

 ciable difference of effect ; and (2) that within this range there 

 is a definite ratio between the stimulus and the amount of the 

 sensation. The upper limit beyond which an increase of external 

 stimulation is not followed by any observable increase in sensa- 

 tional effect, was first observed by Prof. Wundt. The lower 

 limit has been noted by many observers, and it is indicated in 

 almost every physiological text book. Now it does not matter 

 much to us in taking a general view of things, what the limits are, 

 provided we are sure that such limits exist, inasmuch as it indi- 

 cates another element of proof that psychological phenomena, so 

 far as sensation is concerned, occur within certain physical limits. 

 Fechnet's Investigations. 



The next step naturally was to establish the ratio between the 

 magnitude of the stimulus and the magnitude of the sensation. To 

 do this directly is impossible, as any estimation of the amount of 

 sensational effect following a given stimulus would probably be 

 erroneous, because our perceptions are usually qualitative and 

 only rarely, and never absolutely, quantitative. Fechner recog- 

 nised this fact, and he employed for the solution of the problem 

 various methods by which he measured not sensations themselves, 

 but the amount of discriminative sensibility between two sensa- 

 tions produced by stimuli of unequal magnitudes, and he studied 

 the ratio between the difference of weight and the absolute quan- 

 tity of the stimulation. By varying the amount of the stimulus 

 in every possible way, he ^eliminated the chances of error, and 

 arrived at definite results. These results he formulated into a 

 general "psycho-physical law," which maybe expressed in various 

 ways. Mathematically it may be put, that "sensation increases 

 in proportion to the logarithm of the stimulus." Now "loga- 

 rithms increase in equal degrees when the numbers so increase 

 that the increment has always the same ratio to the magnitude of 

 the number." It may be put in another way by saying that "the 

 more intense a sensation the greater must be the added or dimi- 

 nished force of stimulation in order that this sensation undergo 

 an appreciable change of intensity." The mode of arriving at 

 some of Fechner's results may be better understood by an expe- 

 riment which any one can repeat. In the case of muscular sen- 

 sation, suppose two weights A and B : we wish to ascertain the 

 least difference between these perceptible by the muscular sense, 

 say when we lift them in the hand. Let it be so arranged that 

 both weights are composed of different pieces, so that the one 

 may be made less or more than the other at pleasure. If A and 

 B be nearly equal in absolute weight, the person on whom the 

 experiment is made will judge them to be of equal weight. Let 

 weights be now added to B until the difference between A and B 

 becomes perceptible, and as a test, let the weights be again 

 removed from B until, in sensational effect, A becomes again 

 equal to B ; let the same experiment be repeated with weights of 

 different absolute amount, and it will be found that there is a dis- 

 tinct ratio* between the absolute weight and the weight that had 

 to be added to it or taken from it to produce the least perceptible 

 difference of impressionof whatever these weights may be, up to the 

 limit, ofcourse, which I have already noticed. It will always be found 

 that the additional or subtracted weight is one-third that of the 

 absolute weight — a fraction which indicates the degree of inten- 

 sity of the stimulus required to produce the lest perceptible feeling 

 of difference of sensation, and may be termed the constant propor- 

 tional of that kind of sensation. This fraction, in the case of 

 sensibility to temperature, Fechner found to be one-third ; Renz, 

 Wolf, and Volkmann arrived at the same fraction with regard to 

 auditory impressions ; and various observers have found that in 

 visual impressions it is one-hundreth. 



Now the intensity of sensation depends on two conditions : 

 (i) the intensity of the excitation ; and (2) the degree of excit- 

 ability of the sensory organ at the moment of excitation. But 

 suppose the excitability of the organ equal on two occasions, the 

 intensity of the sensation does not increase proportionately to the 

 increase of the excitation. That is to say, suppose we bring into 

 a dark chamber a luminous body such as a candle — it produces a 

 certain luminous sensation ; then introduce a second, third, and 

 fourth — the excitation is double, triple, or quadruple ; but expe- 

 riment shows that the increase in the amount of the sensation 

 is much less ; in other words, let the stimulus increase from 10 

 to 100 times, and from 100 to 1,000 times, the sensation will be 



