Plioto(jr(i)>hlv Rmeai'chen aiul Portraiture '^07 



While tinger-printH are now an accepted form of evidence in our courtH of 

 law — onlya few newly-appointed and yet uninstnicted magiHtratesipieHtioning 

 tlu'ir validity — it is sinpdar that no use lias hitherto In'en made of them in 

 ciuses of doubtful paternity, only vagtie impressions jus to family likeness Ijeing 

 given in evidence or apparently thought of importance. 



Another lecture closely allied to that just discussed \\:i.^ ilie Royal 

 Institution Friday evening tliscourse on January 27, 1 8'j:3. It is entitled "The 

 Ju8t-Percej)til)le Uitierence"'. In this lecture (Jalton starts with a detinition : 



" W(! .set'iii to oui-sclves to belong to two worlds, which arc governed by entirely (lifTereiit laws; 

 the world of feeling and the world of niatttT — the psychical and the physical— whose mutual 

 relations are the subject of the science of Psycho-physics, in which the ju8t>-perceptible 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-perceptiblo ditlerences 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 thri'shold of con- 

 sciousness. It will there be shown how imporUmt a part is commonly playetl by the imagi- 

 nation in producing faint sensations, and how its power on tliose occasions admits of actual 

 mwisurement." (p. 13.) 



(jalton started hy referring to Weber's Law and illustrating its action by 

 an ingenious mechanical model. He placed on an axle a wheel, a logarithmic 

 l^hor ecpiiangular spiral (perpendicular to the axis and with its pole at tne centre 

 I^Bof the axle) and an index-hand marking on a scale the angle turned through 

 ^Hby the axle. All these were accurately balanced, so that they could rest in 

 ^Hauy position of the axle; rouiy:! the wheel was taken a cord carrying a scale- 

 ^Bpan at one end and a counterbalance weight to the pan at the (jther. Round 

 ^Bthe sjiiral was taken a second cord Hxed at one end to the a.xle and carrying 

 ^■a ball at the other. If now a weight be put into the scale-pan, the axle will 

 ^^ rotate until the increasing ray of the spiral provides leverage enough to 

 balance the weight in the scale-pan. The weight in the scale-pau mejiauring 

 the 'stimulus,' the angle turned through by the index-hand measures the 

 sensation'. Galton demonstrated on the model that as the stimulus grew 

 large the increases of sensation were very small. 



"The pi'ogressive increase in the effective length of the logarithmic arm is small at first, but 

 is seen soon to augment rapidly, and then to become extravagant. We thus gain a vivid 

 insight through this piece of mechanism into the enormous increase of stimulus, when it is 

 already large, that is require<l to pro<luce a fresh increment of sensation, and how soon the 

 time must arrive when the organ of sense, like the machine, will break down under the strain 



I rather than admit of l)eing goaded farther. 

 The result of all this is, that although the senses may perceive very small stimuli, and can 

 endure very large ones without suffering damage, the number of units in the scale of sensation 

 is comparatively small. The hugest increase of good fortune will not make a man who was 

 already well off many degrees hap[)ier than liefore; the utmost torture that can be applied to 

 him will not give much greater pain than he ha.s already suffere<l. The experience of a life that 

 ' Proc. Royal Institution, Vol. .\iv, pp. 13-26; Nature, Vol. XLVii, pp. 319-21, 342-4.'). 

 ' If 6 be the effective radius of the wheel, w the weight in scale-pan, W the weight of the 

 I bidl, <^ the angle of the spiral, 6 the angle of the scale and a the linear constant of the spiral, 

 so that its equation is r = n e"'""*, then the principle of moments gives us ic x b= \V x r sin <p, 



I whence 6 - cot d> log - = cot A log „- . — , which is Weber's Law if ^ be read as sensation and 



3y- 



