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CH. XII] the air should be still (for, when in motion, it was converted, according to opinion, into wind), and as one mass; and as this aggregation of the air could be only over smooth surfaces, the outer coat of the eye (the cornea) seemed, by its smoothness, to favour Aristotle's doctrine, that vision is through a medium, and completed, by refraction, at the bottom of the organ. The medium, set in motion, by colour, was said to give motion, by successive impulses, to the air over the cornea, which communicated the impulse to the organ within; and this superseded the doctrine that vision is produced by rays emanating from the eye. Thus, "the air (that over the cornea), in its turn, sets vision in motion;" but the last clause of the sentence is very obscure, and offers, as some commentators have said, "great difficulties." It may, perchance, be a continuation of the analogy and suggest that, as colouring matter acts, successively, until each particle is saturated, so the impulse is transmitted to the cornea, and finally, from it, to the visual faculty within.