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270 emanation of luminous rays from the eye as light proceeds from a torch or lamp; and he ridiculed the notion that vision is precluded in the dark owing to the extinction of those rays therein. It is probable that this theory first led him to adopt a medium and its successive motion, as the immediate cause of vision; as he had accounted for hearing by the propagation of the impulse given to the air by the sonorous body. Aristotle was unacquainted with the structure of the eye; but he was aware, of course, that it contains humours, and these he held to be necessary, not as being aqueous that is elementary but, as being diaphanous, for this property seemed to be as requisite for vision within the eye, as it is for the transmission of light to the eye. It was this assumed succession of action, after impression upon a diaphanous medium, which led to the conclusion that the eye itself must be diaphanous, and, therefore, that the visual power must be somewhere on the inside of the eye; and this is the only approximation to a right knowledge of the retina and its relations.

Note 6, p. 96. It has thus then been said, &c.] The cause of colour being visible is sufficiently obvious from what has been said; but fire was said to be visible both in darkness and in light, owing to its being, as fire, of the nature of the firmament above, which was believed to be fire, or something identical with fire. It may be presumed that the subject was here introduced, in order to notice and account for those luminous appearances,