304   Life and Letters of Francis Galton

use glasses. Also it is clear that the least-discernible difference may be reached at some points long before it is reached at others, or the measure of resemblance would vary from part to part, and ultimately be a measure

Fig. a.

Fig. b.

Fig. c.

Diagram iv.

of only the most unlike parts. If we agree to an average fineness of line, and an average keenness of sight, we shall still be left with the question of scale'. Dealing with the silhouette, Galton remarks that

"All human profiles of this kind, when they have been reduced to a uniform vertical scale, fall within a small space. I have taken those given by Lavater, which are in many cases of extreme shapes, and have added others of English faces, and they all fall within the space shown in Fig. G. [Galton is, working with the distance from the notch that separates the brow from the nose (nasion) to the parting of the lips as standard length.] The outer and inner limits of the space are of course not the profiles of any real faces, but the limits to many profiles, some of which are exceptional at one point and others at another. We can classify the great majority of profiles so that each of them shall be included between the double borders of one, two, or some small number of standard portraits, such as Fig. c. I am as yet unprepared to say how near together the double borders of such standard portraits should be drawn; in other words, what is the smallest number of grades of unlikeness that we can satisfactorily deal with. The process of sorting profiles into their proper classes, and of gradually building up a well-selected standard collection, is a laborious undertaking if attempted in any obvious way, but I believe it can be effected with comparative ease on the basis of measurements as will be explained later on, and by an apparatus that will be described." (p. 174.)

The reader will now be able to perceive better what Galton was really attempting to do by this special illustration: he was aiming at identifying individuals by their profiles, and in order to do this it was needful to index profiles.- This leads Galton to the topics of indexing and of entering indices. He first refers to Bertillon's system of identifying criminals, and states that the actual method by which it is done is not all that theoretically could be desired. He notes a fundamental difficulty that arises

"The fault of all hard-and-fast lines of classification when variability is continuous, is the doubt where to place and where to look for values that are near the limits between two adjacent classes." (p. 175.)

' For example, suppose it be required to find the degree of resemblance between two maps, A and B, of the same district on different scales; shall we reduce A to B, or B to A, for that will clearly affect our judgment? Or, shall we look at them both placed on the table before us, or both hung at some little distance on a wall?