332 Life and Letters of Francis Galton
Galton now gives a paragraph of considerable importance which shows that he had anticipated and met the criticism which naturally arises on
reading his second paper.
"The portions of objects to be compared and between which Resemblance is to be measured must be strictly defined. Non-essentials may be either marked out or be simply ignored, but there must be no vagueness as to the limits of the portions selected for comparison. If the objects be portraits the selected portion may be any specified part of the whole of it. It may be a single feature, it may be the face irrespectively of hair, and of beard if any, it may be the whole head, or it may be the entire person. But, whatever it may be, it must be defined."
After defining the objects for comparison as comparates, Galton. continues
"The comparate is limited to the portion under comparison, the two comparates are supposed to be reduced to similar scales, to be mounted, side by side on the same moveable screen squarely to the line of sight, and to be viewed in a good light through a perfectly transparent atmosphere."
Now in order to conduct the experiments successfully the experimenter requires to have the power of adapting the focus of his eye sharply to the various distances of the screen or to use an optical contrivance to supply this faculty if he should be deficient in it--at the time of writing this paper Galton was 84 years old, and the following words are very characteristic and indicate at least the nature of one of his `toys,' which I had puzzled over:
"The range of adaptability of my own eye, as in that of most elderly persons, has become very narrow, and during a long time was the cause of serious embarrassment in my various experiments on Resemblance. But all this difficulty was happily removed by a small inverting telescope of very low power,, that I made abroad in a very makeshift way, out of two small magnifying glasses that I had by me, with pasted paper tubes and corks. It acted so well that I was loath to replace it by a better. Its field of view was ample and enabled me to focus my eye sharply on 'comparates' at any distance from a few inches upwards. I will call telescopes that neither magnify nor minify by the name of Isoscopes; their use is simply to secure a sharp focus for the eye at any distance. Two convex lenses of 2 inches focal length seem to be on the whole most suitable for an isoscope. The tubes must admit of a wide range of adjustment. Either lens may serve as the eye-piece, but when used as such it should be covered by a cap with an eyehole. Distances must be measured from the object-glass. An isoscope should be fitted with two eye-pieces one of them furnished with a micrometer of crossed lines [i.e. an areal micrometer]. If the eye-piece be of 2 inches focus, and the distances between the lines one 50th of an inch, the intervals between them will subtend 1 sol and each small square will subtend one square-sol."
Galton proposes to take as his index of mistakability the number of square sols covered by either comparate when they are at the `critical distance,' and the corresponding angle is the critical angle. The measure of Resemblance between two comparates, he says, is the angular area of either of them at the critical distance when the comparates as a whole are mutually mistakable. The angular area as a whole is proportional to the number of just-distinguishable plots (i.e. for the normal eye plots of about 1 minute diameter) which they contain, the possibility of mistaking one comparate for another being due to apparent identity in every one of the just-distinguishable plots. The more numerous the plots, the more-minute is the coincidence, and consequently the closer the resemblance. It will be seen that the entire difference between this earlier and the later paper of Galton is the measurement by a solid instead of a plane angle.