[49] Prof. Jastrow has ascertained by statistical inquiry among the blind that if their blindness have occurred before a period embraced between the fifth and seventh years the visual centres seem to decay, and visual dreams and images are gradually outgrown. If sight is lost after the seventh year, visual imagination seems to survive through life. See Prof. J.'s interesting article on the Dreams of the Blind, in the New Princeton Review for January 1888. [50] Impression means sensation for Hume. [51] Treatise on Human Nature, part i. § vii. [52] Huxley's Hume, pp. 92-94. [53] On Intelligence (N. Y.), vol. ii. p. 139. [54] Principles, Introd. § 13. Compare also the passage quoted above, vol. I, p. 469. [55] The differences noted by Fechner between after-images and images of imagination proper are as follows: _After Images._ _Imagination-images._ Feel coercive; Feel subject to our spontaneity; Seem unsubstantial, vaporous; Have, as it were, more body; Are sharp in outline; Are blurred; Are bright; Are darker than even the darkest black of the after-images; Are almost colorless; Have lively coloration; Are continuously enduring; Incessantly disappear, and have to be renewed by an effort of will. At last even this fails to revive them. Cannot be voluntarily changed. Can be exchanged at will for others. Are exact copies of originals. Cannot violate the necessary laws of appearance of their originals--e.g., a man cannot be imagined from in front and behind at once. The imagination must walk round him, so to speak; Are more easily got with shut than Are more easily had with open than with open eyes; with shut eyes; Seem to move when the head or eyes Need not follow movements of head move; or eyes. The field within which they appear The field is extensive in three (with closed eyes) is dark, contracted, dimensions, and objects can be flat, close to the eyes, in imagined in it above or behind front, and the images have no almost as easily as in front. perspective; The attention seems directed forwards In imagining, the attention feels towards the sense-organ, in as if drawn backwards towards the observing after-images. brain.