330 Life and Letters of Francis Golf on 



If we vary n = n' taking a whole series of values for them, we reiich a value 

 N at which the two portraits can be mistaken for |)ortrait8 of the same in- 

 dividual. This value of X is Galton's "index of niistakahility." Two persons 

 will have little resemblance to each other, if they must 1x3 put at a great dis- 

 tance off to l>e mistaken for each other; when they are very like each other 

 the distance will lie small or the index of niistakability will be lai'ge. The 

 index if it can be determined is therefore a mesusure of general resemblance. 

 " Faces," Galton writes, 



"that are alike are certainly [in-Jdistinguishable nt shorter distancps than unlike ones, and I 

 notice no excessive clustering of values closely round fiarticular values of N in my results, 

 which there would be if niistakability always occurre<l near a particular stage, such as that 

 at which the whites of the eyes cease to be visible, or at twice or three times that distance. 

 A strong likeness in small details may so dominate the perception that a want of likeness in 

 larger features is overlooked. Hero the dist-ance of maximum niistakability will be small, the 

 portraits ap})earing more unlike when removed farther off, and the small details ceasing to be 

 visible. Extreme cases of partial likeness, whether in contour or in detail, would of course be 

 noted and allowed for. With these exceptions the index of mistakability appears to l)e a fair, 

 even, as I think, a close, approximation to an index of resemblance when the quality of the 

 observed likeness is recordefl by appropriate letters as will 1)«» <lescril)f»d later. 



The observational value of niistakability lies in its asking; a simple question which different 

 persons would answer in the same way, when they ha<l Ixvonie faiiiiliiir with the method." 

 (p. 562.) 



The difficulty about the di.stance me«asure of mistakability lies in the 

 fact that the comparison of two portraits of different sizes involves continued 

 resetting of the portraits at different distances. To expedite matters Galton 

 tabled a for given it and n, so that the operator knowing ?< and u' could 

 quickly send the two portraits to their pro[>er distances for a given n. Never- 

 theless the continual snifting for each new judgment is laborious. Galton then 

 proceeded to prepare test types and noted the d at which each row of figures 

 was just unreadable. If now a test line be put against the portraits tbem- 

 selves when they are just mistakable in a clear light, we can interchange d 

 and readability of a certain type. By marking the types by bold values of d 

 we replace our distance scale by a type scale. Now if the hindrance to vision 

 increa.ses the portraits with the test card must he brought nearer to the eye, 

 and they will mcrease simultaneously in legibility. The written d will always 

 show wnat the true (/ would be in a clear light. We now see how the 

 "blurrere," wedge or inverted telescope' are to lie used; we can keep the 

 actual d constant, and measure the apparent d on the cjird of test types 

 place<l alongside the portmits. 



(ialton's reduction to test types seems to emphasise an obvious criticism 

 — the judgment of the index of mistakability will lie dependent on the 

 keenness of vision of the operator. Hence different operators would need 

 differently marked test cards, and there would Ije a need to correct the 

 index for personal equation, if the results of operators with marked dif- 



' Atone stage of Galton's experiments he made "blurrers" of gauze of different meshes. 

 I think it likely he discarded these because the visibility so largely depends on the position of 

 the network b(;tween eye and object. Type absolutely illegible if the gauze be midway between 

 eye and object becomes legible if the gauze screen be quite close to object or to eye. 



