Pfiotof/raphic Researches and Portraiture 331 



ferences of keenness of vision Ijad to be pooled. Gulton considers that 

 mutual mistakahility may occur under any one or more of the following 

 conditions, which he thinks should Ije noted alongside the index: 



"aa. Tho portraits ani apparently exact copies or reductioriH on different scalea. 



a. Tli(>y uppear to be portraits of the same pernon at about the Hanie age, though difTering 

 in poxe aii<[ dress. 



l>. Th(«y would in' mistaken for portraits of the same person, even thoUfi^h they differ in hl'X 

 and considerably in nge, if the hair had Ix^en cut and dyed alike, and tli-' il-'.>v< itrrange<l in the 

 same way. 



c. Aa above, if much disjjniswl, as for theatrical impersonations. 



b-c. Applies to cases intermediate between I) and c. 



p. Their resemblance is partial only, being confined to specified features. 



The applications of the process are num<>rou8, as nmst always Ihi the case when a hitherto 

 vague perception is brought within the grip of numerical precision. To myself it has the special 

 interest of enabling the departure of individual features from a standard type to Ije expressed 

 numerically. The departure may l)e from a composite of their race, or from a particular indi- 

 vidual. The shortcomings of a (xxligree animal from a highly distinguished ancestor could be 

 measured in this way. Many other examples might lie given." (p. 563.) 



As in his profile work Galton used a very large number of pairs of photo- 

 graphs of relatives to test his index of mistsikabuity upon. He asked in the 

 newsj)apers for photographs of families, and they appear to have been rained 

 down upon him; some material was suitable, some quite unsuitable! It seems 

 to me that to get reliable measures of resemblance special photographs should 

 be taken — ^full face and profile, the hair being screened under a tight fitting 

 elastic cap. Further if bearded individuals are to be one of the "comparates," 

 then the comparison must further be maile with the chin and lips screened; 

 the eye is very apt to be misled in its judgments by extraneous characters 

 sucli as hair and pose. 



A manuscript typed and prepared for press in February 1906, entitled 

 "The Measurement of Visual Resemblance," seems never to have been pub- 

 lished. It adopts a somewhat different index to that finally chosen by Galton 

 in October of the same year. He begins by saying that visual resemblance 

 between any two objects may be measured in units whose value is strictly 

 defined. 



"Resemblance is independent of actual magnitude and has therefore to be expressed in 

 angular units. It is curious that no popular terms exist to express them in the language of 

 any civilised country, for not only would they be useful, but the diameter of the sun when 

 paled by an intervening screen affords an excellent and practically constant standard for rough 

 measurement. It would often be well to indicate objects in a distjiiit landscape by describing 

 them as so many sun-breadths to the right or left of some conspicuous feature, or to speak of 

 a mountain seen from a specified place as towering so many sun-brejidths in height, or as 

 bulkii\g so n\any [square] sun-breadths in area. But as sun-breadths are not terms in popular 

 use, and as they are not the best unit for the purpose of this memoir, I will employ another 

 that is. The sun's diameter may be taken as subtending an angle of 31 '0 minutes of a degree. 

 T will employ for my unit the diameter of an imaginary mock sun that subtends 34'4 minutes, 

 and is therefore wider than that of the real sun in tho proportion of 10 to 9. Its merit lies in 

 the fact that the tangent as also the arc of 34'4 minutes differ insensibly from O'Ol ; in other 

 words the angular unit is that which is subtended by 1 mcA-sure of any kind, at the distance of 

 100 measures of the same kind. I will call the arc subtended by this angle at any specified 

 distance a 'sol'." 



m 



42—2 



