Photographic Researches and Portraiture 329 (F Measurements of Resemblance.
I have already referred to Galton's long-continued researches on the measurement of resemblance. He gave in Nature, October 4, 1906', some account of his method and of his apparatus2 for measuring his "index of mistakability." He opens his account with the following words
"At the distance of a few scores of paces the human face appears to be a uniform reddish blur, with no separate features. On a nearer approach specks begin to be seen corresponding to the eyes and mouth. These gradually increase in distinctness, until-at about thirty paces-the features become so clear that a hitherto unknown person could thereafter be recognised with some assurance. There is no better opportunity of observing the effects of distance in confounding human faces than by watching soldiers at a review. Their dress is alike, their pose is the same, the light falls upon them from the same direction, and they are often immoveable for a considerable time. It is then noticeable how some faces are indistinguishable at distances where great diversity is apparent in others, and the rudely-defined idea will be justified that the distance at which two faces are just mistakable for one another might serve as a trustworthy basis for the measurement of resemblance. The same may be said of obscurity, of con
fused refractions, and of turbid media."
In the apparatus described in this paper in Nature, Galton used -distance. But he also looked at two portraits through a graded series of "blurrers'," or glasses with different thicknesses of Canada Balsam placed upon them. Finally he adopted a neutral coloured wedge (like the wedge photometer used for star magnitudes), looking at the portraits through thicker and thicker parts of it until they were "mistakable." The apparatus is fairly simple for the distance observations. There is a six-foot base board upon which are two sledges carried along its length by endless cords each going round their own pair of wheels, one at either end of the board. At the summit of the base board, which slopes slightly downwards from the observer, is a screen with an eye-slit to carry spectacle lenses for examining the photographs ; it can be replaced by a bracket upon which optical combinations can be mounted for throwing the photographs to a considerable distance, i.e. greater than that of the base board, in the manner of an inverted telescope. The sledges each carry a standard to which the portraits to be examined can be attached, and when attached they can be rotated in azimuth to compensate for differences of degree in the photographs of "half" face. The position of the photographs with regard to the observer's eye can be read on a scale which runs down the centre of the base board.
Galton's procedure is as follows : He first measures in millimetres the distances u and u' from the pupil line to the lip line of each portrait. He then takes from the base board scale the distances d and d' of the two portraits from the eye screen in centimetres. If now the indices n = 10Ou/d and n'= 100u'/d' be formed, then when they are equal, the two portraits subtend the same angle at the eye, and this allows for any difference in size.
' Vol. LXXIV, pp. 562-3.
2 Now with a good many additions, devised by Galton himself, in the Galton Laboratory.
' He presented me in 1907 with a series of .'°blurrers" and there are a good many sets in the Galtonianc, but I have not come across any account of their preparation and standardisation. The photometric wedge is a much more permanent measurer.