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Photographic Researches and Portraiture   303

all in the matter, and the smallest amend that those who introduced him there can make, is to furnish him with the most serviceable of all information to him, the complete life-histories of all his near progenitors." (p. 31.)

The idea of portraiture as expressing mental character and that of individuality as measured by deviation from type fascinated Galton throughout the whole of his long life, and he returned to,these subjects with great energy even in his last years. He sought to measure the degree of resemblance or of difference in portraits. The amount of labour he put into this research was immense; there is a great mass of manuscript matter, there are endless profiles drawn by his assistants, there are models of apparatus and there is apparatus itself. Without a more definite key than we possess it is often very difficult to trace what line of thought he was following up, although not infrequently one lights on most suggestive ideas in side tracks from the main problem.

That the work in this direction arose from the composite photograph investigations is clear from a lecture Galton gave on May 25, 1888 at the Royal Institution, entitled "Personal Identification and Description'". It opens _with the following words

"It is strange that we should not have acquired more power of describing form and personal features than we actually possess. For my own part I have frequently chafed under the sense of inability to verbally explain hereditary resemblances and types of features, and to describe irregular outlines of many different kinds, which I will not now particularise. At last I tried to relieve myself as far as might be from this embarrassment, and took considerable trouble, and made some experiments. The net result is that while there appear to be many ways of approximately effecting what is wanted, it is difficult as yet to select the best of them with enough assurance to justify a plunge into a rather serious undertaking. According to the French proverb, the better has thus far proved an enemy to the passably good, so I cannot go much into detail at present, but_will chiefly dwell on general principles." (Nature, Vol. xxxviit,

p. 173.)

Galton then states that while recognising different degrees of likeness and unlikeness we have not so far as he knows made any attempt to measure

them. He now proposes to take for his unit of measurement the least-discernible difference.

"The measurement of resemblance by units of least-discernible difference is applicable to shades, colours, sounds, tastes, and to sense-indications generally."

Galton illustrates his method on sight differences; he takes two superposed oval contours (see Fig. a, Diagram iv, p. 304), intersecting one another, and then halves the distance between their. boundaries for a new contour, and 'then halves again until he reaches-in his case in the fourth stage-a contour indistinguishable from one of the original contours. He then says there are 16 grades of least-discernible difference between A and B. The method is suggestive, but obviously liable to difficulties, for it is clear that its measurement is largely subjective. It depends on the fineness of drawing of the original _contours and of the subdividing contours. It depends also on the scale upon which they are drawn. It is modified by the subjective conditions of the observer, whether his sight is good, and whether he uses or does not

' Nature, Vol. xxxviii, pp. 173-77, 201-2,1888; Proc. Royal Institution, Vol. xii, pp. 346-60, 1889; my references are to the pages of Nature.