1893.] Mr. Francis Galton on The Just-Perceptible Difference. 13 



WEEKLY EVENING MEETING, 



Friday, January 27, 1893. 



David Edward Hughes, Esq. F.R.S. Vice-President, 

 in the Chair. 



Francis Galton, Esq. F.P.S. M.B.L 



TJie Just- Perceptible Difference. 



We seem to ourselves to belong to two worlds, which are governed 

 by entirely different laws; the world of feeling and the world of 

 matter — the psychical and the physical — whose mutual relations are 

 the subject of the science of Psycho-physics, in which the just-per- 

 ceptible difference plays a large part. 



It will be explained in the first of the two principal divisions of 

 this lecture that the study of just-perceptible differences leads us not 

 only up to, but beyond, the frontier of the mysterious region of mental 

 operations which are not vivid enough to rise above the threshold of 

 consciousness. It will there be shown how important a part is 

 commonly played by the imagination in producing faint sensations, 

 and how its power on those occasions admits of actual measurement. 



The last part of the lecture will deal with the limits of the power 

 of optical discrimination, as shown by the smallest number of adjacent 

 dots that suffice to give the appearance of a continuous line, and the 

 feasibility will be explained of transmitting very beautiful outline 

 drawings of a minute size, and larger and rougher plans, maps, and 

 designs of all kinds, by means of telegraphy. 



Material objects are measurable by external standards, about 

 which it is sufficient to say that when we speak of a pound, a yard, or 

 an hour, we use terms whose meanings are defined and understood in 

 the same sense by all physicists. The feelings, on the other hand, 

 cannot be measured by external standards, so we are driven to use 

 internal ones, and to adopt a scale of sensation formed by units of 

 just-perceptible differences, rising in the arithmetical order of 1, 2, 3, 

 &c, and by their side a scale of measurements of the stimuli that 

 provoked them. The attempts of those who first experimentalised 

 in Psycho-physics were mainly directed to ascertain the relation 

 between the increase of stimulus and the corresponding increment of 

 sensation. 



Their net result has been to confirm, within moderate limits, the 

 trustworthiness of Weber's law, namely, that each successive incre- 

 ment of sensation is caused by the same percentage increment of the 

 previous stimulus. 



The rate at which a stimulus must be increased in order to give a 



