332 Life and Letters of Francis Galton 



Gallon now gives a paragrapli of considerable importance which shows 

 that he had anticipated and met the criticism which naturally arises on 

 reading his second paj)er. 



"The portions of objects to Ix- coniparetl aiul between which Resemblance is t<j Ix; nieanured 

 must be strictly defined. Non-essentials may be either marked out or he simply ignored, but 

 there must be no vagueness iis to the limits of the portions st>lecte<l for comparison. If the 

 objects be portraits the selecttnl pt)rtion may be any s|x.>cilie»l part of the whole of it It may 

 1k> a single feature, it may be the face irrespectively of hair, and of be«rd if any, it may l)e the 

 whole hea<l, or it may Ik^ the entiri' person. Kut, whatever it may lie, it must l)o defined." 



After defining the objects foe comparison as comparates, Galton continues : 



"Thecompiirate is limited to the portion undi'r comparison, the two comjMvrates are supposed 

 to be re<luced to similar scales, to be mounted side by side on the same moveable screen 

 squarely to the line of sight, and to be viewed in a good light through a perfectly transparent 

 atmosphere." 



Now in order to conduct the experiments successfully the experimenter 

 requires to have the power of adapting the focus of his eye sharply to the 

 various distances of tlie screen or to use an optical contrivance to sujjjjly this 

 faculty if he should be deficient in it — at the time of writing this paper 

 Galton was 84 years old, and the following words are very characteristic and 

 indicate at least the nature of one of his 'toys,' which I had puzzled over: 



"The range of adaptability of my own eye, as in that of most elderly persons, has. become 

 very narrow, and during a long time w^ the cause of serious embarrassment in my various ex- 

 periments on Resemblance. But all this difliculty was happily removed by a small inverting 

 telescope of very low power, that I made abroad in a very makeshift way, out of two small 

 iiuignif\'ing glasses that I had by me, with pasted pii|>er tul>es and corks. It acted so well that 

 I was loath to replace it by a lietter. Its field of view was ample and enabled mo to focus my 

 eye sharply on 'comparates' at any distance from a few inches upwards. I will call telescopes 

 that neither magnify nor minify by the name of Isoscopes; their use is sim])ly to secure a sharp 

 focus for the eye at any distance. Two convex lenses of 2 inches focal length seem to be on 

 the whole most suitable for an isoscope. The tubes must admit of a wide range of adjustment. 

 Either lens may serve as the eye-piece, but when used as such it should be covered by a cap 

 with an eyehole. Distances must be measured from the object-glass. An isoscope should be 

 fitted with two eye-pieces one of them furnished with a micrometer of cros.sed lines [i.e. an areal 

 micrometer]. If the eye-piece be of 2 inches focus, and the distjinces between the lines one 50th 

 of an inch, the intervals Ijetween them will subtend 1 sol and each small w^uare will subtend 

 one square-sol." 



Galton proposes to take as liis index of mistakability the nujuber of 

 square sols covered by either comparate when they are at the 'critical distance,' 

 and the corresponding angle is the critical angle. The measure of Resemblance 

 between two comparates, he says, is the angular area of either of them at the 

 critical distance when the coujparates as a whole are mutually mistakable. 

 The angular area as a whole is proportional to the number of just-distinguish- 

 able plots (i.e. for the normal eye plots of about 1 minute diameter) which 

 they contain, the possibility of mistalcing one comparate for another being due 

 to apparent identity in every one of the just-distinguishable plots. The more 

 numerous the ])lots, the more minute is the coincidence, and conse(juently 

 the closer the resemblance. It will be seen that the entire difference between 

 this earlier and the later paper of Galton is the measurement by a solid instead 

 of a plane angle. 



