238 Life and Letters of Francis Galton
The reader of the paper will certainly realise-probably for the first time -how very varied is the power of mental imagery among individuals. But unless the reader is very familiar with the process of `ranking' he is unlikely to extract at once from that system such results as that 12 per cent. of persons see the mental image as vividly as the real thing, 12 per cent. only recall colours by a special effort for each, and more than 6 per cent. have a larger field of mental than of normal view, i.e. can see more than a hemisphere, all the faces of a die at once or the three walls of a room, and even the fourth simultaneously by an effort. It may be doubted whether the ranking scheme was best adapted to attract attention to a most interesting investigation. The paper concludes with a few observations on "visualised numerals."
I had frequently been puzzled by a number of lantern slides in the Galtoniana, which besides giving various phenomena associated with mental imagery provided illustrations of Bushman, Eskimo and palaeolithic drawings and carvings. They undoubtedly belonged to some public lecture, but there was nothing in the three lists of papers prepared by Galton himself to indicate that this lecture was ever published, nor was there any statement on p. 339 of the Inquiries into Human Faculty to say that the "1880 Mental Imagery, Fortnightly Review; Mind" referred to practically distinct papers. Theyy are, however, distinct, and although the Fortnightly paper, entitled "Mental Imagery," does not cover the whole ground of the lantern slides, there is little doubt that it contains a great deal of the substance of the lecture to which they belonged. The lecture was certainly one on "Mental Imagery," and, although it was not published in extenso, the Fortnightly probably contained the substance of it. There is little doubt that both slides and Fortnightly paper deal with the matter of Galton's popular lecture at the Swansea Meeting of the British Association in 1880. According to L. G.'s Record that meeting was attended by Galton and his wife. Mrs Galton makes no reference to the lecture, nor have I discovered any manuscript of it in the Galtoniana. It is possible that it was needful to cut out a good deal of the material of the lecture from the Fortnightly article as it would not be intelligible without illustrations.
The paper commences with what Galton himself calls vague physiological considerations concerning the difference between a sensation received by the optic nerve and transmitted to the brain, and a mental image where the sequence of events would occur in the reverse order, there being the propagation of a central impulse from the brain towards the optic nerve. This reverse process can be so vigorous that the mental image is vivid, and may in certain cases amount to a hallucination. These considerations
"justify us in ascribing the marked differences in the quality, as well as the vividness, of the mental imagery of different persons, to the various degrees in which the several links of a long nervous chain are apt to be affected." (p. 313.)
Galton states that
"his purpose is to point out the conditions under which mental imagery as above defined is
1 Fortnightly Review, Vol. xxviii, N.S. pp. 312-24, September, 1880. The Mind paper is entitled "Statistics of Mental Imagery." (Brit. Assoc. Report, 1880, p. lxxviii.)