1893.] on The Just-Perceptible Difference. 15 



pain than lie has already sometimes suffered. The experience of a 

 life that we call uneventful usually includes a large share of the 

 utmost possible range of human pleasures and human pains. Thus the 

 physiological law which is expressed by Weber's formula is a great 

 leveller, by preventing the diversities of fortune from creating by 

 any means so great a diversity in human happiness. 



The least-perceptible difference varies considerably in different 

 persons, delicacy of perception being a usual criterion of superiority 

 of nature. The sense of pain is curiously blunt in idiots. It varies 

 also in the same person with his health, and extraordinarily so in 

 hysteria and hypnotism, at which times sensitivity is sometimes 

 almost absent, and at other times exceptionally acute. It is somewhat 

 affected by drugs. Thus Dr. Lauder Brunton writes concerning 

 strychnine, that when taken in small doses for a long time, the im- 

 pressions are felt more keenly and are of longer duration. The sense 

 of touch is rendered more acute; the field of vision is increased, 

 distant objects are more distinct, and the sense of hearing is 

 sharpened. (Pharmacology, 1885, p. 888.) 



Other drugs or intoxicants may yet be discovered and legitimately 

 used to heighten the sensitivity, or indeed any other faculty during a 

 brief period, in order to perform that which could not otherwise be 

 performed at all, at the cheap price of a subsequent period of fatigue. 



Measure of the Imagination. — The first perceptible sensation is 

 seldom due to a solitary stimulus. Internal causes of stimulation are 

 in continual activity, whose effects are usually too faint to be per- 

 ceived by themselves, but they may combine with minute external 

 stimuli, and so produce a sensation which neither of them could have 

 done singly. I desire now to draw attention to another concurring 

 cause which has hitherto been unduly overlooked, or only partially 

 allowed for under the titles of Expectation and Attention. I mean 

 the Imagination, believing that it should be frankly recognised as a 

 frequent factor in the production of a just-perceptible sensation. Let 

 us reflect for a moment on the frequency with which the imagination 

 produces effects that actually overpass the threshold of consciousness, 

 and give rise to what is indistinguishable from, and mistaken for, a 

 real sensation. Every one has observed instances of it in his own 

 person and in those of others. Illustrations are almost needless; 

 I may, however, mention one as a reminder ; it was current in my 

 boyhood, and the incident probably took place not many yards from 

 where I now stand. Sir Humphry Davy had recently discovered 

 the metal potassium, and showed specimens of it to the greedy gaze 

 of a philosophical friend as it lay immersed in a dish of alcohol to 

 shield it from the air, explaining its chemical claim to be considered 

 a metal. All the known metals at that time were of such high specific 

 gravity that weight was commonly considered to be a peculiar 

 characteristic of metals ; potassium, however, is lighter than water. 

 The philosopher not being aware of this, but convinced as to its 

 metallic nature by the reasoning of Sir Humphry, fished a piece out 



