308 Life and Letters of Francis Galton
we call uneventful usually includes a large share of the utmost possible range of human pleasures and human pains. Thus the physiological law which is expressed by Weber's formula is a great leveller, by preventing the diversities of fortune from creating by any means so great a diversity
in human happiness." (pp. 14-15.)
Galton notes how the threshold of sensation differs in different persons and how delicacy of perception is a criterion of a superior nature. It may be modified in the same person by health and disease, by drugs and hypnotism.
He notes, however, that external causes of stimulation may be reinforced by internal causes, and that external stimuli which would fail to exceed the threshold may by aid of the imagination be magnified to the production of a just-perceptible sensation. As-illustration of this Galton quoted a personal experience which certainly deserves record in a biography for it indicates how Galton worked "habitually searching for the causes and meaning of everything that occurred to him'." After citing Wordsworth and Tennyson as cases in which the force of imagination could. master their sense of the present real, Galton notes that his own deafness prevented him when seated in the middle rows at a scientific meeting from following memoirs read in tones suitable to the audience at large. He could, however, distinguish the words of the speaker if he had the unrevised proof of the memoir before his eyes. If the speaker used words not in the proof, he failed to catch them, and if he raised his eyes from the proof nothing whatever of the reading could be understood, the overtones by which words are distinguished being too faint to be understood. He found that he had to approach the speaker by one quarter of his distance from him to follow him without the aid of seeing the words. The loudness of the overtones at the two distances would be as 9 to 16, and Galton concluded that his auditory imagination is to that of a just perceptible sound as 16 minus 9 or 7 is to 16.
"So the effect of the imagination in this case reaches nearly half-way to the level of consciousness. If it were a little more than twice as strong it would be able by itself to produce an effect indistinguishable from a real sound." (p. 19.)
He suggests that experiments as to this might be easily made with two copies of the same newspaper, a few words being altered here and there in the copy to be read from.
People growing deaf, although they cannot lip-read, appear to interpret sounds better when they watch the lips of the speaker. Spectators at the theatre,-e.g. at the French plays, hear better if they follow with a "Book of Words."
Whatever may be thought of Galton's explanation-the internal stimulus due to the imagination-we must recognise that he discovered a most interesting psychological problem in an experience which then bulk of men would never have thought of analysing'.
The next part of Galton's paper deals with optical continuity and the just-perceptible distance between two dots. The ordinary eye is just able or just unable to see two dots about a minute of angle asunder. Taking
1 Charles Darwin: see our p. 1.
2 A somewhat similar experience occurs in deciphering very bad handwriting; we find it impossible to read the words, until we take to imagining what the writer is likely to be talking about, and with this assistance the eye.pan often realise what the hieroglyphics stand for.